wmaker: replace and be replaced (ICCCM protocol)

Use the same logic used by xfwm4, metacity et al to replace an existing
window manager on the screen and allow other window managers to replace
us, as defined by the ICCCM 2.0:

http://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM/icccm.html
  Communication with the Window Manager by Means of Selections

By convention those window managers try to become the selection owner of
the WM_Sn atom where n is the screen number.  If the atom is owned by
another window manager and the --replace argument was not given to wmaker
we fail to start.  If the argument was given we try to become the new
owner and wait for the existing window manger to exit.

After a successful startup we watch for SelectionClear events on the
atom and initiate a shutdown if one arrives, as that implies that
another window manager was started with --replace.

src/screen.h

@@ -69,6 +69,7 @@ typedef struct WDrawerChain {
 typedef struct _WScreen {
     int	screen;			       /* screen number */
     Window info_window;		       /* for our window manager info stuff */
+    Atom sn_atom;		       /* window manager selection */
 
     int scr_width;		       /* size of the screen */
     int scr_height;