Add QWaylandOutput to support multiple outputs

Add a new QWaylandOutput class to support multiple outputs.

Add a primaryOutput() method to Compositor and forward output related
calls from QWaylandCompositor to the primary output.

Rename OutputGlobal to Output and Output to OutputResource. Add support
for physical size. Use better defaults for geometry and refreshRate from
the QWindow if available.

Change-Id: I742996571ddb78328f7bfa4f79b25a81995279e1
Done-with: Jan Arne Petersen <jan.petersen@kdab.com>