Optionally allow point to enter composed character sequences

Optionally allow point to enter composed character sequences

This changeset is based on code originally posted by
Kenichi Handa <handa@gnu.org> as part of discussing bug#20140,
with a few adjustments and changes, mainly to support R2L
text and provide a user option to control the feature.
* src/xdisp.c (compute_stop_pos, next_element_from_buffer)
(composition_reseat_it): Don't allow auto-composing characters
past point when 'composition-break-at-point' is non-nil.
(syms_of_xdisp) <composition-break-at-point>: New boolean variable.
(redisplay_internal, redisplay_window): Disable "just move the
cursor" optimizations when 'composition-break-at-point' is non-nil.
* src/keyboard.c (command_loop_1): Force redisplay when
'composition-break-at-point' is non-nil.

* lisp/cus-start.el (standard): Provide customization form for
'composition-break-at-point'.

* etc/NEWS: Announce the new feature.