Refactor code to separate concerns

This is not a bug, but just a list of possible code improvements that I had written down on paper earlier.

Centralize rule parsing.
Separate properly or merge completely UI and non-UI models.
Move pickCell() and/or allNodes() functions to proper container.
Investigate the dangling cellValue object (might have been addressed already).
Rename some functions/classes for clarity and consistency.

See also #37