Deploy Qt platformthemes plugins with Qt Creator

Makes sure Qt Creator integrates nicely e.g. with gtk.

Task-number: QTCREATORBUG-11406
Change-Id: I4da11b42e9d407ee8456c6d43479839667ee13c1
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>