window: Add initial support for client-side decorations under Wayland

window: Add initial support for client-side decorations under Wayland

This change comprises four main parts:
 * the creation of the widgets that form the decorations,
 * implementation of get_preferred_height/width, and the for_width/for_height
   variants,
 * taking the decorations into account when allocating,
 * and drawing the decorations themselves.

Kristian did the bulk of the original work on this but any bugs are almost
certainly mine through the many refactorings and rebasings.