Levels of difficulty, screen ratio

Done:
1) Levels of difficulty - Finished and tested
Easy - units generated with no limits
Medium - max 2 units of the same color in one new row/column
Hard - all squares in the same new row/column have different colors

2) Adjusted the height multiplier for the different screen ratio like
16:9 and others. By default the game runs on ratio 16:10.

3) Created empty classes for other screens like splash screen, menu
screen and others.