gdk: Generate crossing events around touch devices' press/release

gdk: Generate crossing events around touch devices' press/release

Anytime a touch device interacts, the crossing events generation
will change to a touch mode where only events with mode
GDK_CROSSING_TOUCH_BEGIN/END are handled, and those are sent
around touch begin/end. Those are virtual as the master
device may still stay on the window.

Whenever there is a switch of slave device (the user starts
using another non-touch device), a crossing event with mode
GDK_CROSSING_DEVICE_SWITCH may generated if needed, and the normal
crossing event handling is resumed.