Get pointer button mapping from input system

Get pointer button mapping from input system

If XInput extension is available, then find the (first) pointer device
and use the button mapping of that one. If there are more than one
pointer devices, they could have different button mappings, but it is
not clear how this should be communicated to the RDP server.

If XInput is not available, attempt to fallback to the old global
mapping. (This mapping exists, but is not correct if there actually
is an XInput extension loaded, as it is then not used).