ENH: wait_cursor

[GuillaumeFavelier]

This PR changes the current cursor to Qt.WaitCursor, effectively the "spinning wheel" mentioned in #308, during a data_changed task. I went with data_changed because I thought it would be more impactful but maybe it's not the best place.

Here is how it looks for me:

output.mp4

Closes #308

[GuillaumeFavelier]

This is ready for reviews @cbrnr

[cbrnr]

This looks great @GuillaumeFavelier! At least on KDE (and GNOME I assume), but on macOS it shows an ugly b/w cursor (https://interest.qt-project.narkive.com/nFRAp9hx/very-old-wait-cursor-on-mac-osx, https://bugreports.qt.io/browse/QTBUG-25249). It actually looks as expected in main, because after a short delay the spinning beachball appears.

I'd like to see how this looks on Windows (I can test this). Meanwhile, could you special-case macOS so that the busy cursor is not shown when running on that platform? Depending how it looks on Windows, we will add Windows to that list (or not).

[GuillaumeFavelier]

Sure, I'll make it a no-op on macOS 👍

[cbrnr]

OK, so I tested this on Windows and it doesn't have any influence the way it is right now. So I guess this now only works on KDE/GNOME, but there I think it is a great feature!

Do you want to add a changelog entry?

[GuillaumeFavelier]

This is ready to go from my end @cbrnr

[cbrnr]

Thanks @GuillaumeFavelier!