qhelpgenerator: Fix warning about qpa_minimal_plugin usage

  Project WARNING: CONFIG+=qpa_minimal_plugin is deprecated. Use QTPLUGIN.platforms=qminimal instead.

Change-Id: I9f07c5033e0022e00c45e52570fe001856f13ca8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
winrtrunner: Split AppxEngine into abstract base and Appx Local subclass

This allows common code to be reused in future Appx engine types.

Change-Id: I4e0ba09242eee6687604abe65d556a96a79346f4
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>