GDK W32: Handle drivers that do not send WT_CSRCHANGE after WT_PROXIMITY

GDK W32: Handle drivers that do not send WT_CSRCHANGE after WT_PROXIMITY

Some drivers don't do that (not sure whether that is the correct behaviour
or not). Remember each WT_PROXIMITY with LOWORD(lParam) != 0 that we get,
then look for a WT_CSRCHANGE. If WT_CSRCHANGE doesn't come, but a WT_PACKET
does, assume that this device is the one that sent WT_PROXIMITY.

Also include fallback code to ensure that WT_PACKETs for an enabled device
disable the system pointer, because WT_PROXIMITY handler might have
enabled it by mistake, since it's not possible to know which device left
the proximity (it might have been a disabled device).

https://bugzilla.gnome.org/show_bug.cgi?id=778328