wayland: Move Wayland-specific code out of AbstractWaylandOutput

Currently, output properties are looked up either on the wl_output
object or the output device object. This puts a hard dependency on the
wayland server in the platforms.

This change intends to fix some flaws in the current output
abstractions, and allow creating/destroying wayland-specific globals as
we wish.

With the work done in this patch, the need for the AbstractWaylandOutput
class is unclear, and it might be a good idea to merge it with the base
AbstractOutput class.