state: Add xkb_state_key_get_consumed_mods

state: Add xkb_state_key_get_consumed_mods

This retrieves the mask of consumed modifiers for a given key and state,
which is helpful for toolkits without having them to do it one modifier
at a time, or pass in 0xFFFFFFFF to xkb_state_remove_consumed_mods to
"reverse-engineer" the consumed mods.