QOpenGLContext: Use static invocation of QGuiApplication::platformNativeInterface().

Fix MSVC warning (release build):
qopenglcontext.cpp(1116) : warning C4189: 'app' : local variable is initialized but not referenced

Change-Id: I00fa5237bbac4c0e3bb63ea9d3e5096e05dbe1be
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Windows: Return false from event processing of unhandled multimedia keys.

Otherwise, potentially active players no longer receive the keys when
a Qt application is running.

Task-number: QTBUG-43343
Change-Id: Iefa511a101734690305e3244fafec4a460a9212d
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>