touchpad: revamp thumb detection

Instead of a simple yes/no/maybe for thumbs, have a more extensive state
machine that keeps track of the thumb. Since we only support one thumb anyway,
the tracking moves to the tp_dispatch struct.

Test case changes:
touchpad_clickfinger_3fg_tool_position:
  with better thumb detection we can now handle this properly and expect a
  right button (2fg) press for the test case
touchpad_thumb_no_doublethumb_with_timeout:
  two thumbs are now always two fingers, so let's switch to axis events here

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