The aim is to be able to create a plugin for each of the backends.
The following directories are created:
* backends/drm
* backends/fbdev
* backends/wayland
* backends/x11
Requires to move the screensQueried signal and disconnect during
Application startup.
As Screens only hold the geometry of each screen and the overall
geometry, there is no need (yet) to update in more detail when a
DrmOutput is added or removed. The overall queried is sufficient.
Introduces a new (optional) dependency: libdrm.
The DrmBackend currently supports finding the first connected output.
It can create shared memory buffers which are used by SceneQPainter to
do double buffered rendering.
There is still lots to do, the following things are not yet working:
* multiple outputs
* page flip
* OpenGL (through gbm)
* restoring mode setting to start value
