panel: Add transparency when free-floating

panel: Add transparency when free-floating

The solid black top bar we currently use works well for maximized
or tiled windows, as it puts focus on the application content by
blending into the monitor bezel. However it also visually reduces
the screen space, which is particularly noticeable when no window
is located nearby. Having the top bar blend with the background is
a better option in that case, so track window positions and add
some transparency when the top bar is free-floating.

https://bugzilla.gnome.org/show_bug.cgi?id=747163