(x_display_cursor): Set XIC pre-edit area only if window

(x_display_cursor): Set XIC pre-edit area only if window
is its frame's selected window.
(xim_instantiate_callback): Likewise.

(XTread_socket) [HAVE_X_I18N]: If event is for none of
our frames, call XFilterEvent with 2nd parameter `None'.
(XTread_socket) <KeyPress> [HAVE_X_I18N]: Handle XmbLookupString
returning XBufferOverflow.
(XTread_socket) <ConfigureNotify> [HAVE_X_I18N]: Set XIC status
area.
(x_display_cursor) [HAVE_X_I18N]: Set XIC pre-edit area.
(x_new_fontset) [HAVE_X_I18N]: Create XIC status area fontset.
(xim_destroy_callback, xim_open_dpy, xim_instantiate_callback)
(xim_initialize, xim_close)
[HAVE_X_I18N && HAVE_X11R6]: New functions.
(x_destroy_window) [HAVE_X_I18N]:  Call xic_destroy_frame.
(x_term_init) [HAVE_X_I18N]: Call xim_initialize.
(x_delete_display) [HAVE_X_I18N]: Call xim_close.