The previous approach was to remember the last event and flush it at the right
time. The new approach is to update the device state during the frame and send
out the events at EV_SYN time.

This gives us two advantages: we are not dependent on the kernel order of how
events come in and we can process events depending on other events in the same
frame. This will come in handy later for button debouncing.

This is also the approach we have in the touchpad and tablet backends.

Two FIXMEs are left in place, the button debouncing code and the lid switch
code. Both need to be handled in future patches.

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