keymap, symbols: improve xkb_key memory layout

keymap, symbols: improve xkb_key memory layout

Add struct xkb_group and xkb_level for use in xkb_key, to mirror how
it's done in KeyInfo, GroupInfo, LevelInfo in symbols.c. This
corresponds more nicely to the logical data layout (i.e. a key has
groups which have levels), and also removes a lot of copying and ugly
code due to the index indirections and separate arrays which were used
before.

This uses more memory in some places (e.g. we alloc an action for every
level even if the key doesn't have any) but less in other places (e.g.
we no longer have to pad each group to ->width levels). The numbers say
we use less overall.

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