winrt: Correctly check if mouse button was clicked

The idea behind this code was to check, whether no
button was pressed. !isPressed only checks, whether
XButton2 was pressed though.

Change-Id: I358816fa62d230abf82116f0da7bc3a5e43fbaf6
Reviewed-by: Andre de la Rocha <andre.rocha@qt.io>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>