Let VirtualOutput be a child class of the new generic class Output.
This allows code sharing and a very similar behavior of the Virtual backend
in comparision to the Drm backend.

Autotests succesful with one exception: The decoration input test fails on
testDoubleTap, row topLeft. This is to be expected because now the
ScreenEdgeInputFilter captures the event at position (0,0) before the
DecorationEventFilter can capture it. The autotest therefore needs to be
adapted to take this special case into account.