server: add wl_input_device_release()

Add a clean-up function for destroying all objects created in
wl_input_device_init(). Can be used to fix memory leaks reported by
Valgrind in the demos.

The init function was also missing an explicit initialisation of the
'keys' array. Add the explicit array init, although it is redundant with
the zeroing of the whole struct.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

krh: Edited to rename function to *_release()