mtdev is used only for MT Protocol A device of which there are quite
few. But that protocol is also a perfect example for event frames in ->
different event frame out so let's move this into the plugin pipeline.
Because the plugin doesn't really have full access to the device's
internals we set up mtdev base on the libevdev information rather than
just handing it the fd and letting it extract the right info.
A minor functionality change: previously mtdev-backed devices returned
zero on libinput_device_touch_get_touch_count(). Now it is hardcoded to
10 - the number of callers that care about this is likely near zero.
Because it's now neatly factored out into a plugin we can also make
mtdev no longer a strict requirement.
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1245>
