Add support for _NET_WM_STATE_SHADED

Add support for _NET_WM_STATE_SHADED

* doc/lispref/frames.texi (Management Parameters): Document new
frame parameter `shaded'.
* etc/NEWS: Announce new frame parameter.

* src/frame.c (frame_parms) [HAVE_X_WINDOWS]: New frame
parameter `shaded'.
(syms_of_frame): New symbol `shaded'.
* src/xfns.c (x_frame_parm_handlers): Install handler
`x_set_shaded'.
* src/xterm.c (x_set_shaded): New frame parameter handler.
(x_get_current_wm_state): New parameter `shaded'.  All callers
changed.
(x_net_wm_state):
(x_handle_net_wm_state): Handle shaded state.
(x_term_init): Intern new atom `shaded'.
* src/xterm.h: Update prototypes.