Introduce special value __focused__ for criterion con_id.

This allows matching with

    [con_id=__focused__] unmark

for commands that do not default to operating on the focused window
if no criteria have been specified (such as unmark).

relates to #2014

docs/userguide

@@ -1664,7 +1664,8 @@ con_mark::
         mark.
 con_id::
 	Compares the i3-internal container ID, which you can get via the IPC
-	interface. Handy for scripting.
+	interface. Handy for scripting. Use the special value +\_\_focused__+
+	to match only the currently focused window.
 
 The criteria +class+, +instance+, +role+, +title+, +workspace+ and +mark+ are
 actually regular expressions (PCRE). See +pcresyntax(3)+ or +perldoc perlre+ for