Allow non square sub-blocks, add 3x2 game mode

android/gradle.properties

assets/files/templates.json

assets/icons/size_2.png

assets/icons/size_2x2.png

assets/icons/size_3.png

assets/icons/size_3x2.png

assets/icons/size_3x3.png

generator/batch.sh

generator/generate.py

icons/build_game_icons.sh

icons/size_2.svg

icons/size_2x2.svg

icons/size_3.svg

icons/size_3x2.svg

icons/size_3x3.svg

lib/entities/cell.dart

lib/layout/board.dart

lib/layout/game.dart

lib/layout/parameters.dart

lib/provider/data.dart