Set BTN_TOUCH, BTN_TOOL_DOUBLETAP automatically depending on the number of
fingers down.

This emulates real event sequences a bit better than the current approach,
though it's not a 100% correct emulation:
1) On real devices, BTN_* are usually sent last before the SYN_REPORT - here
   they are sent first to slot in with the custom, device-specific event
   sequence. We should only ever look at the complete sequence anyway, so this
   shouldn't matter.
2) On real devices, the switch from BTN_TOOL_DOUBLETAP to TRIPLETAP and vice
   versa is not always toggled within the same SYN_REPORT
3) On synaptics devices, BTN_TOUCH is released in the frame where
   BTN_TOOL_DOUBLETAP is set. It is then immediately set again in the next
   frame.  With the current litest framework this is hard to integrate, so we
   just leave BTN_TOUCH set the whole time, which is what MT devices do if
   they don't have BTN_TOOL_DOUBLETAP.

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