Display pre-edit information from X input methods

Display pre-edit information from X input methods

This also repurposes the `pgtk-preedit-text' event to be
meaningful on X, renames it `preedit-text', and documents it.

* doc/lispref/commands.texi (Misc Events): Document
`preedit-text'.
* lisp/term/pgtk-win.el (pgtk-preedit-text): Bind to
`preedit-text' instead.
* lisp/term/x-win.el (x-preedit-overlay): New variable.
(x-preedit-text): New command, bound as a special event to
`preedit-text'.

* src/keyboard.c (kbd_buffer_get_event):
(make_lispy_event): Rename PGTK_PREEDIT_TEXT_EVENT
PREEDIT_TEXT_EVENT.
(syms_of_keyboard): New defsym `preedit-text'.
* src/pgtkterm.c (pgtk_enqueue_preedit): Use PREEDIT_TEXT_EVENT
instead.
* src/termhooks.h (enum event_kind): Rename
`PGTK_PREEDIT_TEXT_EVENT' `PREEDIT_TEXT_EVENT'.

* src/xfns.c (Xxic_preedit_draw_callback):
(Xxic_preedit_caret_callback):
(Xxic_preedit_done_callback):
(Xxic_preedit_start_callback): New callback variables.

(STYLE_OFFTHESPOT, STYLE_OVERTHESPOT):
(STYLE_ROOT, STYLE_CALLBACK, STYLE_NONE): New macros.
(supported_xim_styles): Use reasonable values.  This also serves
as a better fix for bug#10867.
(best_xim_style): Restore code deleted as part of the original
fix for bug#10867.
(create_frame_xic): Add preedit callbacks.
(xic_set_preeditarea): Add preedit callbacks.
(x_xic_to_frame):
(xic_preedit_start_callback):
(xic_preedit_caret_callback):
(xic_preedit_done_callback):
(x_xim_text_to_utf8_unix):
(xic_preedit_draw_callback): New functions.

* src/xterm.c (x_detect_focus_change): Fix type of XI event.
(x_free_frame_resources): Free preedit text buffer if still
present.
* src/xterm.h (struct x_output): New fields `preedit_size',
`preedit_chars' and `preedit_active'.