Summary:
This is the first change in a refactoring series. The aim is to:
* rename AbstractBackend to Platform
* move backends/ to plugins/platforms/
* don't bind platforms to Wayland only
* provide a platform plugin for "normal" X11
* share more code between X11 and Wayland

This change moves the platform/backend from waylandServer to Application.
The init of the plugin happens directly in the Application from the
KPluginMetaData. There is no need to externally init it and set the
parent.

WaylandServer::backend() currently just delegates to
kwinApp()->platform(), the idea is to drop this method completely.

The test infrastructure is also adjusted to this change.

Test Plan: kwin_wayland still works, all tests pass

Reviewers: #plasma, sebas

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1331