Fix coding systems used for X input methods

Fix coding systems used for X input methods

* doc/emacs/mule.texi (International): Refer to X Coding as
well.
(Communication Coding): Document that locale-coding-system is
not always used on X to decode keyboard input.
(X Coding): New node.
* etc/NEWS: Announce change to input method coding resolution.
* lisp/term/x-win.el (x-get-input-coding-system): New function.
* src/coding.c (syms_of_coding): Update doc string of
locale-coding-system.
* src/xfns.c (struct x_xim_text_conversion_data)
(x_xim_text_to_utf8_unix_1, x_xim_text_to_utf8_unix_2)
(x_xim_text_to_utf8_unix): Accept dpyinfo.  Use the coding
system specified inside if possible.
(xic_preedit_draw_callback): Pass dpyinfo.
* src/xterm.c (handle_one_xevent): Use XIM coding system for IM
input.
(xim_open_dpy): Try to determine the input method coding system.
(mark_xterm): Mark `xim_coding'.
(syms_of_xterm): New variable `x-input-coding-system'.
* src/xterm.h (struct x_display_info): New field `xim_coding'.
(FRAME_X_XIM_CODING): New macro.