Support multimedia keys on Windows

We currently do not receive events from most multimedia keys on Windows
because they are sent using appcommand events instead of the normal key
events.

This patchs adds support for parsing appcommand events.

Change-Id: I19f568b6fa0d3d136e3a22318df1499a1fab6edb
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>