layout toggle: take any combination of layouts as arguments (#2649)

With this PR the 'layout toggle' command can be passed any
combination of valid layout keywords as arguments. They will
be activated one after another each time you issue the command,
advancing from left to right always selecting the layout after
the currently active layout or the leftmost layout if the active
layout is not in the argument list.

This PR also incorporates the feature request from #2476.

parser-specs/commands.spec

@@ -110,7 +110,7 @@ state LAYOUT:
 state LAYOUT_TOGGLE:
   end
       -> call cmd_layout_toggle($toggle_mode)
-  toggle_mode = 'split', 'all'
+  toggle_mode = string
       -> call cmd_layout_toggle($toggle_mode)
 
 # append_layout <path>