A signal is added to the VirtualBackend to change all screen geometries.
This can be used from auto tests to change the screens. But it's not yet
correctly reflected in other areas. E.g. not forwarded to Wayland Output,
etc.
Summary:
Source code reorganization:
The base class AbstractBackend got renamed to Platform, thus the
"backends" are "platforms" now. As they are plugins they should go
together with other KWin plugins which are nowadays in the folder
plugins.
So new location is plugins/platforms/
Reviewers: #plasma, sebas
Subscribers: plasma-devel
Projects: #plasma
Differential Revision: https://phabricator.kde.org/D1353
