Support for toggling between horizontal/vertical tab placement

Added support for toggling between horizontal and vertical tab modes.
Currently it supports only left and top tab placements.

The current behaviour is as follows :

* Expands all the tabs when switching from vertical tab placement to
horizontal tab placement.
* Restores the tab hierarchy when switching back to vertical tab
placement.

fix for issue #2 
ok, so the problem seems to be that:
when a component is hidden in Swing, it disappears from the layout as well. There is no way to "just" hide it.

I found an answer on StackOverflow: use a cardlayout to create Hideable components. Implementing that would be better than this hack.