Easier back jumps in Basic Movement Training (fixes
bug #692)
The explanation of Back Jumping (2/2) has been simplified
and the "hard" part has been made easier by lowering the girders.
The original idea was that I wanted to force players to learn
how to jump higher by delaying the 2nd backspace keypress.
But this turned out that this section was too unfair and we have
lost at least one player due to rage-quitting, according to feedback.