Baseline for a unified touchpad driver

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
doc: build doxygen documentation, add a state diagram for tapping

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Add support for two-finger scrolling

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Improve documentation to something mostly useful

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>