Dragging monitors in the display preferences left ghost images of the
previous position on systems without a compositor.

This patch removes creating an entire drawing context on the parent
window just for testing when edges touch and instead use a single-pixel
cairo surface. That way we avoid interfering with pending redraws. Then
we replace the manual canvas-to-window coordinate translation for
invalidation with a simple redraw call, which should handle windowless
widget positioning correctly.

Fixes #808

mate-appearance-properties only looked for a gtk-2.0/gtkrc file when
deciding whether a theme was installed, so GTK3 themes were reported as
not installed.

This adds a monitor for the gtk-3.0/gtk.css file to validate whether the
theme is GTK3.

Fixes mate-desktop/mate-themes#276