touchpad: Add support for top softbutton area

touchpad: Add support for top softbutton area

Add support for the top softbutton area found on some laptops.

For details of how this works, see the updated
doc/touchpad-softbutton-state-machine.svg diagram.

Basically this mirrors the state-machine for the bottom softbutton area, with
one exception, if a finger stays at least inner timeout milliseconds in the
top button area and then moves out of it, it will be ignored rather then
become the pointer. This is done so that people using the top buttons together
with a trackstick and accidentally move their finger out of the upper area
don't get spurious pointer movements from the finger on the trackpad.

This behavior is indentical to xf86-input-synaptics, which also ignores
movements from touches which start in the top button area.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>