Add support for EWMH extended frame synchronization

Add support for EWMH extended frame synchronization

* src/xfns.c (Fx_create_frame): Populate both counter variables.
* src/xterm.c (XTframe_up_to_date):
(handle_one_xevent): Support extended synchronization.
(x_free_frame_resources): Destroy extended counter.
(x_term_init): Intern new atom _NET_WM_FRAME_DRAWN.

* src/xterm.h (struct x_display_info): New atom
`_NET_WM_FRAME_DRAWN'.
(struct x_output): New fields for extended counter tracking.
(FRAME_X_EXTENDED_COUNTER): New macro.