added random push delay for rocks, nuts and bombs to EM game engine

added random push delay for rocks, nuts and bombs to EM game engine

Just like in the original Emerald Mine game, there is now a random
delay when trying to push a rock, a nut or a bomb. While the chance
for rocks being successfully pushed at first attempt will be around
50 %, the chance for nuts will be a bit lower than that for rocks,
and the chance for bombs will be a bit lower than that for nuts,
with a roughly similar overall delay as in the original game.