Gdk win32: set active source device where needed

This could fix some issue people are having with Wintab. Wintab code sets the active source device (even though Wintab devices are floating, not slave) but the mouse handling code never sets the active source device back to system_pointer.

Also add a source_device argument to send_crossing_event() and related functions. This is just a preparation for the Windows Pointer Input Stack MR.