evdev: add minimal support for touchpads

The added support is very very basic (single touch, motion only,
no acceleration, no pressure recognition), but anything more
complex requires a state machine that will be hopefully provided
by libinputcommon in the future.
And at least, with this patch the pointer moves, which will be
useful for people testing wayland in 3.10 without a physical mouse.

https://bugzilla.gnome.org/show_bug.cgi?id=706494