Make xkb_keymap_num_leds return the index range instead of active count

Make xkb_keymap_num_leds return the index range instead of active count

Currently xkb_keymap_num_leds() returns a count of valid (settable)
leds. Because the indexes might be non-consecutive, and some leds
might not be settable, it is incorrect to use this function for
iterating over the leds in the keymap. But this is the main use case of
this function, so instead of the current behavior we adapt the function
to the use case by making it return the needed range of iteration.
The caller needs to handle invalid intermittent indexes, though.

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