Minor changes to Render;

- Render the cursor if the interface is focussed.
- Use correct range operator to remove requirement to alter value of #viewport_height.
- Add temporary defaults.
Add Area class.

Conflicts:
	lib/vedeu/output/area.rb
Started working on Viewport concept for scrolling interfaces.
Add #refresh method to update the interface after the cursor moves.
Experiment with options.
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.
Combine Offset into Area;

Offset was suffering from feature envy; everything that it needed was found on the Area class.
Rename #indexed_(x|y) to #(x|y)_indices.
Update Cursors;

- Use interface top and left for default starting coordinates for the cursor when adding.
- Rename #cursor method to #build.
- Update documentation.
Rename method to start with coordinate.
Move and reorder methods.
Remove (x|y)_offset from Cursors.