Add cmake test for QtCompositor and disable cmake files for QtWaylandClient

Change-Id: Id6e2d46b55e521218a3c0f44cf9a4a2be5f0df7f
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Fix a few project warnings

Change-Id: I342b677f529994430125797f1b971f8723e1355a
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Fix creation of custom shells

The previous virtual method initShell() was not working because
it was called from QWaylandCompositor constructor.
Replace that with a mthod to create the default shell to be
manually called by the compositors if they so choose.

Change-Id: I35a1dc0edfaf4237ca47b532645ac0d95752311c
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>