When composited, the tab popup uses an OSD style which typically has a
dark background, so we use a light background highlight color.
However, when not composited the popup uses a light color and should
thus use a dark highlight.  This was done for the workspace popup, but
not for the window one, leading to the highlight being hardly visible
in several themes.

This is Metacity commit 50358a95 ("Allow applications to raise windows
when raise_on_click is off. Closes #445447.") applied to Marco without
modification.

It also includes a change to the GSettings key's description to remove
the now obsolete warning on the raise-on-click key, & replaces it with
an actually useful description. This is copied from the equivalent key
in gsettings-desktop-schemas.

Fixes: https://github.com/mate-desktop/marco/issues/762

Enabling code that is supposed to be used in compositing conditions is
harmful if compositing is not actually available. Just checking the
preference is not enough to make sure that compositing is available -
the X server might be missing crucial extensions for compositing to
work, which in turn correctly disables the internal compositor.

The end result is graphical issues like black borders around windows in
such situations.

Make sure that compositing is both requested AND available to fix this
bug.

Fixes #757.