window: rework the CSD theming layer

window: rework the CSD theming layer

Instead of having three different boxes and style classes, we can just
get away with the regular background box, plus a window-frame, which
contains the external frame, together with the window drop shadows.

GtkWindow now has special code to ensure the backing actual window is
allocated big enough to accomodate the shadows (using the shadow size
calculations introduced in the previous commit). We also use the margin
value to determine the size of the invisible borders (which can then be
different than the shadow).