Re-added the CellPanel state.  It's just easier.  And really, in the grand scheme of things, there was no really great reason for removing it.  In fact, it was probably a little misguided.
Changed the visibility of the classes that arne't API facing to protected.
Setting state on the Cell object again.
Changed the names of the x and y coordinate fields to avoid confusion.
Deletion tool now takes a reference to Level.
Tests update due to changes made to the DeletionTool.