GtkCssProvider: Factor out a function

Move finding the gtk.css file into its own function. We will
make this more complicated in the near future.
GtkCssProvider: Load themes from versioned directories

Look for themes in $prefix/themes/$name/gtk-$version/ (for
version = 3.16, 3.14, ...), before using the old location
$prefix/themes/$name/gtk-3.0/. This gives theme authors a
way to support multiple versions of GTK+ 3 with separate
css files.
mir: actually pass the GdkMirScreen to display config change cb
mir: unset config change callback on finalize
mir: correctly get screen mm sizes
mir: clear screen data
mir: don't crash when querying infos for an invalid monitor
mir: clear screen objects correctly