symbols: Restrict the number of actions and keysyms per level In preparation to support capitalization in `xkb_state_key_get_syms()`, this commit reduces the number of supported actions and keysyms per level, going from UINT_MAX to UINT16_MAX. This is most likely still more than enough and could be even reduced further, but deemed unnecessary at the moment: alignment of `struct xkb_level` is driven by the fields `a` and `s`. - Switched the item count type from `unsigned int` to `uint16_t`. - Introduced `xkb_{action,keysym}_count_t` type for the respective item count for exact typing. - Added relevant bounds checks.