Introducing QPlatformHardwareCompositor.

Can be used by applications to send hardware buffers directly
to the hardware compositor, if available.

Change-Id: Ibdcdcc744c34869d3abbc11aad448a755f87161f

Conflicts:
	src/gui/kernel/kernel.pri
	src/gui/kernel/qplatformintegration.cpp
	src/gui/kernel/qplatformintegration.h