Move QPlatformInputContextPlugin/Factory to QtGui

Move QPlatformInputContextPlugin/Factory to QtGui

PlatformSupport is no longer shared and there should be no plugin
factory in it otherwise it will break plugin loading.

Since PlatformInputContext is already in QtGui, so move QPlatformInput-
ContextPlugin/Factory to resolve the platforminputcontexts plugin
loading problem. And remove platformsupport-private dependency from
existing inputcontexts plugin.

Change-Id: If4cb766470b8f9c8b72157da86cb33b4e1ff09e3
Reviewed-by: Pekka Vuorela <pekka.ta.vuorela@nokia.com>