input: add MT_TOOL_DIAL

A dial is a tool you place on a multitouch surface which reports its
orientation or a relative angle of rotation when rotating its knob.

Some examples are the Dell Totem (on the Canvas 27"), the Microsoft Dial,
or the Griffin Powermate, though the later can't be put on a touch surface.

We give some extra space to account for other types of fingers if we need
(MT_TOOL_THUMB)

Slightly change the documentation to not make it mandatory to update each
MT_TOOL we add.

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>