Made QQuickWindowManager pluggable via QSGContext.

Also rename paint() to exposureChanged() as that is what the function
actually means. The implementation of paint() has been removed in both
trivial and threaded version as it is superflous as show() already
triggers rendering.

Change-Id: I7e53f42786efaf961921f10a39977de511965d71
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>