First stabs at Clickable refactoring

This is a work in progress, but should work as is.

- 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)
- oh this also already includes some renaming, e.g. tl --> text_layout for readability

Also includes a little fix to the problem that no errors were raised (in slot.rb) so it is actually due to the URL sub system.

What's missing for a merge here?

- some sort of tests, although those may be hard 
- figuring out a solution so that Link would not have to be treated specially.. e.g. a backend class but that could also be a separate Pull Request