Implement inputMethodQuery

This makes it possible on touch devices to use a simple
virtual keyboard input method that synthesizes key events.
This is the minimal requirement to make the the boot2Qt
Keyboard component functional.
For more advanced input methods and the widget tests we will
need to override inputMethodEvent as well.

Change-Id: If9228ee6b1730d72e7424bdb33a9a9c46654507f
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>