Until now KScreen was loading the backend plugin into each KScreen-enabled
application. This patch changes the design completely: The KScreen library
now requests a backend (which is represented by a DBus interface) from
BackendManager (a replacement for BackendLoader). BackendManager will then
start (if not running) kscreen_backend_launcher process (BackendLauncher),
that will detect the correct plugin for the current platform, load it, and
will create a DBus service called org.kde.KScreen.Backend.%backendName%
with org.kde.KScreen.Backend interface. The library will now communicate
with the backend through this DBus interface.

The out-of-process design solves many issues, including blocking calls in
main thread, better performance (backend aggregates notifications before
emitting them to clients, reducing number of "changes" clients have to handle,
and better stability (backend crash does not crash the application).