evdev: retrieve udev tags

evdev: retrieve udev tags

udev already tags the devices by opening each of them and analyzing their
features. We are basically re-doing this in libinput.

The advantage of udev tags over the plain heuristic from libinput is that
users (or driver writers) can force some tags that are not detected by
common rules. For instance, the pad part of the Wacom tablets is difficult
to discriminate from a joystick or a pointer.

For now we tread INPUT_ID_KEY and INPUT_ID_KEYBOARD as equivalent. It may
become necessary to separate them later.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>