…visible

The visibilitychanged slot currently updates the title regardless of the visibility. We are seeing a crash when the viewer is shut down and a signal is sent to the visibilitychanged slot.  That executes some code which causes a segfault in the python process[1].  This PR changes the visibility changed slot to only update the title bar if the widget is visible.

[1]
```
  * frame #0: 0x0000000115cc96e4 QtWidgets`QWidgetPrivate::reparentFocusWidgets(QWidget*) + 356
    frame napari#1: 0x0000000115cb99cf QtWidgets`QWidget::setParent(QWidget*, QFlags<Qt::WindowType>) + 911
    frame napari#2: 0x0000000115cb82c0 QtWidgets`QWidgetPrivate::init(QWidget*, QFlags<Qt::WindowType>) + 688
    frame napari#3: 0x0000000115d73e5e QtWidgets`QFrame::QFrame(QFramePrivate&, QWidget*, QFlags<Qt::WindowType>) + 14
    frame napari#4: 0x0000000115dc3705 QtWidgets`QLabel::QLabel(QWidget*, QFlags<Qt::WindowType>) + 277
    frame napari#5: 0x0000000116503189 QtWidgets.abi3.so`Sbk_QLabel_Init(_object*, _object*, _object*) + 1497
    frame napari#6: 0x000000010015f84d Python`wrap_init + 12
    frame napari#7: 0x0000000100113e64 Python`_PyObject_FastCallDict + 143
    frame napari#8: 0x00000001001b17a2 Python`call_function + 439
    frame napari#9: 0x00000001001aa46b Python`_PyEval_EvalFrameDefault + 3078
    frame napari#10: 0x00000001001b1ee2 Python`_PyEval_EvalCodeWithName + 1638
    frame napari#11: 0x00000001001b281b Python`_PyFunction_FastCallDict + 447
    frame napari#12: 0x0000000100113e99 Python`_PyObject_FastCallDict + 196
    frame napari#13: 0x0000000100113fa3 Python`_PyObject_Call_Prepend + 131
    frame napari#14: 0x0000000100113d1e Python`PyObject_Call + 101
    frame napari#15: 0x000000010015f7d1 Python`slot_tp_init + 57
    frame napari#16: 0x000000010015c782 Python`type_call + 178
    frame napari#17: 0x0000000100113e64 Python`_PyObject_FastCallDict + 143
    frame napari#18: 0x00000001001141fa Python`_PyObject_FastCallKeywords + 97
    frame napari#19: 0x00000001001b17a2 Python`call_function + 439
    frame napari#20: 0x00000001001aa4fb Python`_PyEval_EvalFrameDefault + 3222
    frame napari#21: 0x00000001001b28e0 Python`_PyFunction_FastCall + 110
    frame napari#22: 0x0000000100113e99 Python`_PyObject_FastCallDict + 196
    frame napari#23: 0x0000000100113fa3 Python`_PyObject_Call_Prepend + 131
    frame napari#24: 0x0000000100113d1e Python`PyObject_Call + 101
    frame napari#25: 0x000000011126f146 libpyside2.abi3.5.14.dylib`PySide::SignalManager::callPythonMetaMethod(QMetaMethod const&, void**, _object*, bool) + 534
    frame napari#26: 0x000000011126eb77 libpyside2.abi3.5.14.dylib`PySide::SignalManager::qt_metacall(QObject*, QMetaObject::Call, int, void**) + 519
```

* Initial experimental performance monitoring

If NAPARI_PERFMON is defined we use our own QApplication
that times events and sends them to our new PerfTimers
class.

If NAPARI_PERFMON_TRACE_PATH is defined them PerfTimers
will write the timers to a chrome://tracing support log file.

NAPARI_PERFMON also creates a new dock widget QtPerformance
which is very much a WIP. It shows the average UpdateRequest time
which the main paint/draw event that hangs if we are doing IO
during the draw. It also has a text window where it "lots" any
"slow" events (UpdateRequest or otherwise).

* Cleanup, remove unused code, remove print

Create LONG_EVENT_MS class var
Remove print and replace with comment.

* Remove stale comment

* Better comments

* Improve comments

* Set max/min not needed

* perf_timers and tracing comments

Big comment on the lack of full nesting
support for PerfTimers

* Define a perf_counter_ns() to support Python 3.6

* Cleanup QtPerformance, add "uptime"

Add "draw time" and "slow events' labels.
Add lots of comments.
Extract method: _get_timer_info

* Better comment describing the layout

* Add perf_timer context object

* Big iteration of perfmon stuff, put in own utils/perf folder (#1)

* A lot of cleanup/improvement to perf stuff
* Move USE_PERFMON check to event_loop.py
* Split things into mutiple files
* New Debug menu and trace file recording menu options.

* Remove stray method

* use perf.USE_PERFOMON instead of env var

* Add a combo box to set threashold (#2)

* Fix decorator

* Need category, placeholder for now

* New perfmon that works with Jupyter (#3)

New convert_app_for_timing works with Jupyter

The old approach was restricted to gui_qt which is only
used when running napari from the command line.

Now we have 2 approaches. We still do gui_qt for the
console app. But in Viewer we call convert_app_for_timing
and this works for Jupyter.

* Fix comment

* Better classname

* Better comment about QApplication

* Add event to string name mapping for PyQt5

In PySide2 the str() method gives you a nice string name like
'PySide2.QtCore.QEvent.Type.KeyPress' But in PyQt5 it return
a string/integer like '6' which is annoying.

This creates a lookup table on startup. This should be 100% find and
perform fine. So we use it for both PyQt5 and PySide2 to be consisting
so we work the same in both.

* Better comment on convert_app_for_timing call

* Edits for readability. Plus add sys.argv to the QApplication()

* Merge in perf_func/perf_func_name from other branch.

Most of the time we can just use perf_func that uses the function name
by default. Most of the time that's what we want.

* Talley review: moves/fixed Parameters for __init__

* Talley review: allow docking to the right

I just picked "bottom" early on before I knew what was going to be in the
widget. Works fine on the right so yes allowing that here.

* Talley review: use env var directly and late import

Avoid importing perf at all unless we are really using it.

* Better comment

* Talley review: remove allowed_areas entirely

Rollable instead of n_moveable dims