(adjust_intervals_for_insertion): Handle insertion

(adjust_intervals_for_insertion): Handle insertion
between two unlike intervals via merge_properties_sticky.
(merge_properties_sticky): New function.
(graft_intervals_into_buffer): Leave handling of `sticky'-ness to
adjust_intervals_for_insertion, then merge properties of the
inserted text onto the old ones.
(textget_direct): New function.
(set_point): Fix calculating of fromprev.
(verify_interval_modification): Check for `read-only' property
and take its `sticky'-ness into account.

(set_point): Ignore `invisible' property unless property value is `hidden'.