Many code improvements to Mine Digger.

Text styles are now a global constant instead of being initialised dynamically.
There are now sufficient text styles for 8 mines around a square.
I coallesced Game and MineDiggerApp.
Used an enum instead of constants for the cell state.
Used setState() instead of scheduleBuild().
Used an O(N) algorithm for mine laying instead of the previous potentially-never-ending algorithm.
Moved the listener on the cells to the app, so that the logic survives a rebuild.
Various other minor changes.