Implement xdg-decoration-unstable-v1 support for toytoolkit

This closes #541 .

Because clients/window.c already contains code to hide the window frame when a window is fullscreened, server-side decorations can be implemented by adding a new condition to use this pathway.

changed the description

added Clients label

Hmm. Currently Weston doesn't implement decorations, and toytoolkit isn't really meant to be an example of ... anything good really. The current thinking is to delete most of it. So I think this probably doesn't belong?

This MR was, in part, a test run to figure out how to get Weston (as a nested Wayland client) to support xdg-decoration, so I'm not particularly attached to it.