state: use darray for filters

state: use darray for filters

For the darray we need to specify the explicit struct xkb_filter type
instead of void*, so we move the definition of struct xkb_state into
state.c thus making it opaque even from the rest of the files. It has
enough getters to get going and is otherwise good style.

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