Improved ToolItem implementation

Known issues: 
* It uses internal API - it should rather use default lookup API if possible
* Missing default constructors (lookup via indexes,...)
* Sub-item menu support ref - https://github.com/jboss-reddeer/reddeer/issues/34
* Combine API with ToolBarMenu (it should be one implementation regardless ToolItem has menu or not)

I understand ReferencedComposite as "component inside of which lookup could be done" and Shell fits this description :-D Correct me if I'm wrong.