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.

man/i3.man

@@ -44,6 +44,9 @@ Retrieve the i3 IPC socket path from X11, print it, then exit.
 Limits the size of the i3 SHM log to <limit> bytes. Setting this to 0 disables
 SHM logging entirely. The default is 0 bytes.
 
+--replace::
+Replace an existing window manager.
+
 == DESCRIPTION
 
 === INTRODUCTION