Since both output-management-v2 protocol implementation and the rest of
kwin live in the same place and the fact that kde-output-management-v2
is very plasma specific, we can move Platform::requestOutputsChange() to
the implementation of kde-output-management-v2 protocol, it simplifies
the code a bit and improve code encapsulation.

In order to further simplify kde-output-management-v2 protocol, this
change alters the behavior of the protocol so an output configuration
can be applied only once, which is a very reasonable behavior.