Updating the documentation for ActionTree.
Also added missing default documentation for the getValue() method.
Simplified the documentation in ActionTree.
Fixed the contribution so that an ActionTree with no default Action and
no children is disabled, as specified in the aforementioned
documentation.
Restructured the ActionTree tester a little bit. It's much more
straightforward, and works like a charm.
Added a context Menu with the same Actions that are in the ToolBar.