Added support for composite widget approach

Now we have support for Shell and Workbench (view, editor) to be the composite widget parent from which discovering chidren widgets is placed. 

Additionally we want to have such a support for other such a widgets which can be recognized as composite parent, e.g different types of groups (textgroup, checkboxgroup ...). 

Note that this approach is also preparation for Eclipse UI Forms support. In Eclipse Forms there are some widgets, which has to be managed in this approach (I defined above) to be able to correctly select some Eclipse Forms widgets.

With new approach you will be able to do the following:

new DefaultShell(); // composite widget parent, support for this widget is already in reddeer
new TextGroup(); // set composite widget parent to first text group in shell
new TextGroup(); // set composite widget parent to first text group in previous text group
new DefaultText(); // find text widget whose parent is text group (the second one in parent-child hieararchy).