WinRT QPA: Improve key handling

Simplify key handling by providing a cleaner tracking of physical key
presses and associated character events.

Change-Id: I5aa8990e0b24e101b348c04d1ada2cbcd1b0b6be
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>