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.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