This lifts the enablement code for outputs from the DRM backend to Platform
allowing other Wayland backends in the future to use this interface as well.
To do that we also create some helper functions on Platform level and have to
spill some KWayland classes into AbstractOutput what motivates a further split
of Platform into a Wayland child class like for AbstractOutput.
This is just a port. There are some inconsistencies with the current logic and
in contrast to before the image of a disabled output is not cleared. I assume
this is not an error in the porting but was before rather accidentally since
there was no actual modeset requested after disabling.