Fix some duplication between QGuiApplication and QApplication,

Fix some duplication between QGuiApplication and QApplication,

for the mouseButtons and keyboardModifiers vars and methods.
Implement queryKeyboardModifiers with a new virtual in
QPlatformIntegration.

Task-number: QTBUG-11243
Change-Id: I9e95841542ac61c73ff72d7682ad962ea8aada42
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>