Implement support for the WM_Sn selection (#4374)

Closes #536

When the WM_Sn selection is already owned at startup, this now either
errors out or waits for the old selection owner to exit.

include/i3-atoms_rest.xmacro.h

@@ -21,4 +21,5 @@ xmacro(I3_FLOATING_WINDOW) \
 xmacro(_NET_REQUEST_FRAME_EXTENTS) \
 xmacro(_NET_FRAME_EXTENTS) \
 xmacro(_MOTIF_WM_HINTS) \
-xmacro(WM_CHANGE_STATE)
+xmacro(WM_CHANGE_STATE) \
+xmacro(MANAGER)

include/i3.h

@@ -39,6 +39,7 @@ extern bool debug_build;
 /** The number of file descriptors passed via socket activation. */
 extern int listen_fds;
 extern int conn_screen;
+extern xcb_atom_t wm_sn;
 /**
  * The EWMH support window that is used to indicate that an EWMH-compliant
  * window manager is present. This window is created when i3 starts and