QtWaylandClient: Pluginize window decorations.

Convert to a D-pointer, split between abstract base class and an implementation.

Also move implementation of the current built-in decoration to the "bradient"
plugin, named in glorious memory of the programmer-designed blue gradient that
will forever sear our eyeballs.

The decoration plugin may be specified using the environment variable
QT_WAYLAND_DECORATION.

Change-Id: Idc99ab06ae138ad299bad2b62b9595379bd007ab
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>