wncklet: center previews when scaling is enabled

[balazs-endresz]

Turns out the off-by-one issue when scaling is enabled can be easily fixed by getting the pointer position as a double: https://docs.gtk.org/gdk3/method.Device.get_position_double.html

This PR also includes https://github.com/mate-desktop/mate-panel/pull/1348 to make testing easier but let me know if you'd prefer otherwise.

I've tested this with scaling, grouping, and multiple monitors in various positions.

[lukefromdc]

Hoping we can get a second reviewer to have a look at this. It's much nicer looking over here, but I am one user testing on one distro, with one monitor.

[zhuyaliang]

I am interested in this. I will test it later