(x_detect_focus_change): Handle embed client message.

(x_detect_focus_change): Handle embed client message.
(handle_one_xevent): Ditto.
(handle_one_xevent): If embedded and we get a button press/release,
request focus.
(xembed_set_info, xembed_send_message): New functions.
(x_make_frame_visible): Call xembed_set_info if embedded.
(x_make_frame_invisible): Call xembed_set_info if embedded.
(x_term_init): Initialize Xatom_XEMBED.
(x_make_frame_visible): Check for FRAME_X_EMBEDDED_P also.
(x_iconify_frame): Ditto.