First stabs at Clickable refactoring

- determination of whether a click is inside moved to the objects
  themselves (#in_bounds?)
- wrote a general Listener class fixing the occasional error
  output (modification fo Java eigenclass) and therefore maybe
  performance issues
- some code still offers some compatibility for mixed GUI/DSL
  objects as Link is a dsl object and everything else is a GUI
  objects... which would have to be fixed (e.g. make an
  SWT Link)
Fix no cursor showing when hovering a clickable element
Refactoring of mouse cursor handling/handling clickable elements

* also added the hidden? accessor which is more idiomatic for
  queries than hidden (e.g. for getting booleans)
* and the accompanying visible?
ln --> click_listener
Some specs for Clickable :-)
removed debbugging output in the URL code
Merge pull request #290 from PragTob/refactorClickable

Clickable refactoring