Field and Engine Refactoring

1. Field class - revised:
* Identifiers names' changed;
* Rows, Cols and Matrix props implemented;
* Zapochni method goes in the Engine class as StartNewGame method;
* Boom method goes in Update and CheckIfIsMine methods;
* Clear method implemented;
* Private methods for validating input coords and calculating adjacent
mines count implemented;

TODO: Documentation

2. Engine class - revised:
* StartNewGame method added and implemented;
* All the source code concerning the Field class/object revised;