touchpad: add support for clickfingers

On touchpads without physical buttons, the number of fingers on the touchpad
at the time the physical click happens decides the button type. 1/2/3 fingers
is handled left/right/middle.

We also swallow the motion event on the actual click event, this reduces
erroneous motion events by a bit. More processing is needed here though.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
touchpad: mark the first finger as pointer-controlling finger

Unused at the moment, but will be used later to determine if a finger should
trigger motion events.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
touchpad: Support finger-pinnnig during physical button presses

On a clickpad, one finger has be on the trackpad to trigger a physical button
press. For drag and drop, we still want motion events though when a second
finger is down.

This patch adds finger-pinning. If the touchpad is pressed, the pressing
finger is "pinned" and ignored for further motion events. A second finger may
then be used to drag.

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