This commit makes it possible to use client-side decorations for
override-redirect windows by calling _gtk_window_request_csd()
before realizing the window. Since the wm won't do interactive
resizing for us in this case anyway, don't bother creating
the border windows we use for this purpose on regular toplevels.

To make this accessible to themes, we set a "csd" style class
on client-side decorated windows. With this, .window-frame.csd.menu
can be used to define the shadow for csd menus, and .menu can be
used to define a border for menus under non-composited wms.

https://bugzilla.gnome.org/show_bug.cgi?id=731187