Add common xkb_key struct

Add common xkb_key struct

Instead of having a million arrays from the keycode to various
key-specific info in the keymap, add a single struct xkb_key to hold all
of the data for the key in one object. This way we can pass it around,
do some refactoring and make the code simpler. It's also nice to see
everything in one place.

The keys array is still indexed by keycode, which is suboptimal because
there may be a lot of holes (i.e. unused keycodes between min_key_code
and max_key_code). By the end of this series it would be abstracted
enough to replace it by a hash table or similar if there's ever a need.

Signed-off-by: Ran Benita <ran234@gmail.com>