Widen keycode range to 8/255 if possible (bug #63390)

If the keycode range is smaller than 8 → 255, artifically widen it when
dumping the keymap as not to displease X.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Add keycode min/max and iteration API

Add three new pieces of API:
  - xkb_keymap_min_keycode does what it says on the tin
  - xkb_keymap_max_keycode likewise
  - xkb_keymap_key_for_each calls the provided function once for every
    valid key in the keymap

Signed-off-by: Daniel Stone <daniel@fooishbar.org>