With the cause of the segfault out of the way, a cleaner implementation of building the mouse submenu can be done.

I am trying to move away from the use of ThreadsafeTimer in SignalProxy, while maintaining backwards compatibility.
Possibilities:

1. create another class (e.g. SignalThrottler) that uses QTimer and change all library uses to this new class.
   • adding yet another public class
2. add a parameter threadSafe=True to SignalProxy and change all library uses to adding threadSafe=False. (this is what is currently implemented in this PR)
   • unwieldy but putting the burden on library developers doesn't seem too onerous
3. create a new class SignalProxyThreadsafe and bind it to pg.SignalProxy. Change SignalProxy to use QTimer. Thus all library uses will end up using the QTimer version, while external users that use pg.SignalProxy still use the ThreadsafeTimer version.
   • having behavior change depending on how you import it doesn't sound good
4. ignore backwards compatibility and just switch to QTimer.

With the change of ubuntu-latest to ubuntu-22.04, the Linux / Python 3.9 / PySide6.2 pipeline segfaults.
Testing on py38, py39, py310 with PySide6.2 showed segfaults on py38, py39 (but not py310).
UPDATE: pyside6.1 and pyside6.0 also has segfaults.

Fixed by skipping test_busycursor and test_progressdialog also on older PySide6.

Thanks for your work on this @pijyoi I know this one was a doozy to sort out.