extras: Add input_id

input_id probes input/event devices for their class (keyboard, keys, mouse,
touchpad, tablet, joystick). This is based on the corresponding hal code in
hald/linux/device.c, input_test_{abs,rel,...}.

This should provide enough functionality to get hal-less X.org working (which
in particular needs to know exactly which devices are touchpads).

Replace the brittle hacks in 60-persistent-input.rules with checking for the
new ID_INPUT_* flags. This keeps the old ID_CLASS properties for now (but they
are to be removed later on).

Note: The current code has several hacks still, which are to be replaced with
proper libudev calls later on.