qpa: Merge qapplication_qpa.cpp into qapplication.cpp

The variable 'appFont' was removed as it was not used.

Change-Id: I8dfa8382b3f30b72490fd22b4e0a27e991318a9c
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
qpa: Rename qwidgetwindow_qpa_p.h to qwidgetwindow_p.h

Change-Id: I24835b86194653e89c0bacefd22ddbff06b6e97b
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>