Export QPlatformIntegrationFactory to make it usable outside of QtGui.

Export QPlatformIntegrationFactory to make it usable outside of QtGui.

QPlatformIntegrationFactory can be used to load a platform plugin dynamically.
It can be useful for an external platform plugin that want to define some
behaviour on it's own and use the default plugin for the rest. We are going
to implement such a plugin in WebKit for the test infrastructure to tweak
the platform backends in order to provide more platform independent layout
test results.

Change-Id: Iaaabe1cbc173a03132d83474faa7858aead4cae0
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>