Ensure editors are removed when replaced

Ensure that editor panel views are removed when
the user swaps editor types, instead of merely layering another
panel on top of them.
Ensure sudoku cells are added under the grid view

By layering the new cells underneath the old ones when
swapping, we ensure new cells are not on top of the grid.
Fix bug in loading puzzles from other puzzles

Fix a bug causing Puzzles to fail to load with the
contents of other puzzles.
Implement KakuroGridGenerator

Create a class that can generate a KakuroPuzzle
with a grid, but no sums.
Add button to display generated kakuro grid

Add a Generate Grid button to the kakuro editor
panel that allows a user to generate a random kakuro
grid.