xdg-decoration: Add client side decoration hints

There's situations where a client side titlebar doesn't make sense. This is e.g. the case on mobile where toplevels are always maximized by the compositor and (if the titlebar doesn't contain any useful UI elements) only takes away precious screen estate. It'd be nice to have an event that informs clients about this situation (which can change, e.g. when an external display is attached). (Currently clients try to detect this situation from XDG_CURRENT_DESKTOP and other settings).

The xdg-decoration protocol looks like a good fit to add that information. I opted for a positive flag titlebar that should be sent by default by all clients implementing version 2 and left out when not titlebar is required. This reads a bit better than the negative variant no_titlebar or skip_titlebar.