Remove the QWidget compositor example

The QWidget API isn't well fit for a Wayland compositor, and does
not have any advantage over a QWindow, which can also draw using
QPainter. Besides the current implementation doesn't work properly,
and no one seems to be interested in fixing it.

Change-Id: Id1c337506af48e1d1fdd8d14f0ed637d299702f3
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>