Follow-up of 4a2a5dc092814bfe ("eOS theme: Add single button backgrounds")

Without this fix these PNGs are installed when using autotools but not using meson.

When the active workspace is changed, marco figures out which window
should get focus by calling `focus_ancestor_or_top_window`. In some
cases that call might include a window that should not get focus as
`not_this_one`.

When `not_this_one` refers to a window, the function will check to see
if it has a parent, and if it does, it will ignore the new workspace's
`mru_list` and will focus that parent window. However, it doesn't check
to see if the parent window is actually on the new workspace. If the
parent isn't on the new workspace, focusing it will drag it over,
including the transient window that was supposed to be ignored. This
isn't the result a user would likely expect, and is made more confusing
by the parent window being minimized, stuck that way until the user
switches to another workspace and back.

This change makes `focus_ancestor_or_top_window` ignore the
`not_this_one` window if it isn't on the new workspace's `mru_list`.
Instead it will just search the new workspace's `mru_list` for a window
to focus.

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.