Implement _NET_WM_USER_TIME protocol

Implement _NET_WM_USER_TIME protocol

* src/xterm.c (x_display_set_last_user_time): Set user time on
the active window.
(handle_one_xevent): Only set user time when the user actually
interacted with Emacs.
(x_make_frame_visible): Set user time if non-zero.
(x_term_init): Intern `_NET_WM_USER_TIME'.
* src/xterm.h (struct x_display_info): New field
`Xatom_net_wm_user_time'.
(x_display_set_last_user_time): Implement in xterm.c instead.