This PR implements a cairo based renderer for Qt canvas, under the name "Qt4Cairo"/"Qt5Cairo". Test e.g. by setting the MPLBACKEND environment variable to qt4cairo/qt5cairo.

The quoted text below is no longer relevant (the corresponding PRs have been merged).

The first three commits include a number of cleanups to specific backends code, some of them related to the issue at hand and some of them not so much. They are also the object of a separate PR (#8772), that may be merged first (and independently) to simplify review.

The fourth commit ("backend class to factor out common code") implements a class-based approach to define the various functions needed by a backend module, in order to remove large chunks of copy-pasted code. It does not modify the public API though. Likewise, it is the object of a separate PR (#8773) that may be merged second to simplify review.

The following commits implement the Qt5Cairo backend, then the Qt4Cairo backend, and redistributes relevant code chunks between the various backend_qt* modules in order to maximize code reuse, and then update the docs. I would suggest restricting comments for this PR to the diff of these last commits.

As a side effect of the refactoring of the Qt canvas classes, this also closes #8618 (PySide2 support).

Note that because of the way the cairo backend is currently written, all the drawing occurs during the Qt draw event, instead of being buffered before it as in Agg. The third party mplcairo backend works around this limitation, but this PR is still useful to cleanly separate the windowing and rendering parts of the code.

PR Checklist

• Has Pytest style unit tests
  -> see test_backends_interactive.py
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/whats_new.rst if major new feature
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way