KWin does not require a decoration plugin any more

With config option "NoPlugin" in group "Style" it is possible to
no request that KWin should be run without a decoration plugin.
This is a change for Plasma Active to properly support no decorations.

As well if a decoration plugin cannot be loaded, KWin will no longer
exit, but continue to run just without a decoration. As a nice
side-effect changes in Oxygen no longer destroy my kwin build ;-)


FIXED-IN: 4.7.0