Added level-score explanation

Underneath the level score there is a short line with icons and numbers that try to explain what influences the score. It's still cryptic but it's a first step. 

Currently, there is no explanation what scores what, e.g. a shot costs 1, undo the same, touching one ball gives 1, activating a house gives 5 points, etc. In the following screenshot I had 2 shots, 1 undo, 1 house and once touched one ball -> 2*(-1) + 1*(-1) + 1*(+5) + 1*(+1) = +3