[platforms/virtual] Let VirtualOutput inherit Output

Summary:
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.

Test Plan:
Autotests succesful with two exceptions: 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 was adapted to take this
special case into account.

Also the lockscreen test fails, because the virtual output is currently missing
the physical size yet.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11789