Render the cursor if the interface is focussed.
Use correct range operator to remove requirement to alter value of #viewport_height.
Add unused defaults.
Add Area class.
Add tests for Area.
Deal with zero heights and widths better.

Conflicts:
	lib/vedeu/output/area.rb
Started working on Viewport concept for scrolling interfaces.
Add Content class to track a virtual cursor within the content of the interface.
Add #refresh method to update the interface after the cursor moves.
Experiment with options.
Add Viewport class.
Experimental stuff for scrolling, cursors and geometry.
Remove unused code.
Simplify by removing empty check and using #key?

- Whitespace.
Add more documentation, also;
- Fix whitespace.
- Remove commented out code.
Move exceptions into correct file.
Make #y and #x public.
Add Offset class;

Calculates offset for a cursor based on an Area and a Cursor.
Remove unused exceptions and methods, also;

- Extract methods and reduce complexity.
Simplify Area by using attributes rather than arguments.