Start adding event stuff
Simplify event handling

The Minefield class has been refactored to send only one event.
I will profile this at some point to figure out what kind of
event granularity I actually need.
Remove random field from Minefield.java
Add isGameOver() method to Minefield.State
Refactor model classes

Instead of a Square objects, Cursor objects are now
returned from Minefield as the primary means of
querying the Minefield.

TODO: get access stuff right.