keysyms: Add min and max assigned keysyms internal API Currently there is no direct way to know the minimum and maximum keysym values that are assigned, i.e. that have an explicit name or are Unicode keysyms. Introduce the new following internal API: - XKB_KEYSYM_MIN_ASSIGNED - XKB_KEYSYM_MAX_ASSIGNED - XKB_KEYSYM_MIN_EXPLICIT - XKB_KEYSYM_MAX_EXPLICIT - XKB_KEYSYM_COUNT_EXPLICIT Also add a bunch of tests to ensure consistant keysyms bounds.