Allow controlling the X input method style

Allow controlling the X input method style

* doc/emacs/xresources.texi (Table of Resources): Document new
resource `inputStyle'.
* etc/NEWS: Announce new resource.

* src/xfns.c (create_frame_xic): Give the display info to
`best_xim_style'.
(best_xim_style): Take the display's preferred style into
account.

* src/xterm.c (x_term_init): Parse the preferred style if
inputStyle is set.

* src/xterm.h (struct x_display_info): New field
`preferred_xim_style'.

(STYLE_OFFTHESPOT, STYLE_OVERTHESPOT, STYLE_ROOT):
(STYLE_CALLBACK, STYLE_NONE): New macros.