Set macOS icon when using Qt backend

[joelfrederico]

PR Summary

On most platforms, windows have their own icons that are displayed in conjunction with windows. On macOS, icons are used differently. Individual windows should not have an icon unless they are associated with a file. On the other hand, processes ("Apps") have icons that can be set.

@greglucas suggested applying these fixes within derivations of FigureManagerBase. FigureManagerBase should only be instantiated when Matplotlib is responsible for running the GUI and its event loop. In this case, it makes sense for Matplotlib to control the App icon. (see #14930 (comment))

PR Checklist

Tests and Styling

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).

[jklymak]

I can confirm this works. Is there a GUI app we can run that will show this doesn't stomp on whatever icon they are showing?

[joelfrederico]

@jklymak Oh, no, it definitely stomps on the icon if you run it via something that uses a FigureManagerBase. If you want to verify it doesn't always stomp on the icon, you just need to run an example of embedding. I'll see if I can find something...

[greglucas]

This looks good to me and works on linux and mac as expected for me, I don't have a Windows box to test on.

Just one minor comment about whether the if blocks are necessary.

[greglucas]

Is there a harm in setting this regardless of platform? I just tested on a Linux box and it didn't seem to have an impact.

Ditto to the block below, where even if we are on mac, I don't think it hurts anything to call setWindowIcon on the windows.

[joelfrederico]

I don't have a problem removing this one.

With the other one, theoretically window icons on macOS are supposed to only be set along with a file path when the window represents a file. However, if you don't set the file path, the icon won't show. So while it's an unnecessary call that's semantically incorrect, it's effectively a noop.

So I guess it's really up to your priorities: semantics? Performance? Less code? Up to you!

[joelfrederico]

Here's the documentation from Qt re: QWidget::setWindowIcon():

https://doc.qt.io/qt-5/qwidget.html#windowIcon-prop

[jklymak]

Does it make sense to keep Darwin for now, and if someone wants this on another system they can add it?

[greglucas]

This looks good to me and works well for me locally. I don't have a huge preference one way or the other on the if blocks, so I'll approve it as-is and someone else can comment if they feel it should be a different way.

[jklymak]

I'll approve as-is If someone wants to add support beyond Darwin, they can do so, but this is the more conservative change...