Don't excessively sync in some other code

Don't excessively sync in some other code

* configure.ac (USE_XCB): Remove xcb-util dependency.
* src/frame.h: Remove x_sync.
* src/gtkutil.c (xg_frame_restack, xg_update_scrollbar_pos)
(xg_update_horizontal_scrollbar_pos): Call XSync manually
instead of x_sync.
* src/xfns.c (x_sync): Delete unused function.
* src/xterm.c (x_send_hourglass_message): New function.
(x_show_hourglass, x_hide_hourglass): Avoid XSync in these two
pieces of frequently used code.
(handle_one_xevent): Handle hourglass messages.
(x_make_frame_invisible): Stop using x_sync.