Hide only the elements of a slot, not its parent contents

* fixes #904
Remove shared togglable spec from flow and add reference to #905
Merge with master - herp derp tobi forgot to pull on this PC before working
toggle => update_visibility for clarity and module name

* The usage of toggle in the visibility code and the inverse logic
  (that hide had to set hidden to false, then call toggle because
  would inverse it and then notify the GUI) was bugging me for
  some time. It worked well, but it was tricky to understand and
  reason about. Furthermore, the backend code that was named
  "toggle" only set the visibility based on the DSL. Nothing was
  toggling there so the module and method_name were inappropriate
  to start with :-) I think this makes it much easier to read :-)
* sorry for the many changes - renames and stuff. But RubyMine did
  ans awesome job supporting me!
Fix crash for Border when hiding

* Changelog: bugfix
* through including Common::Visibility
simple-slide.rb now works thanks to these changes :-)
Adding a simple visibility sample to showcase hide/show/toggle
Merge pull request #906 from shoes/slot-hide

 Hide only the elements of a slot, not its parent contents 

* Changelog: bugfix
* fixes #904
* Also includes a refactoring for more clarity in the toogle/visibility world