"Time" concept implemented

Each Entity has a set_round_cooldown() method, that, when called,
sets a cooldown. The cooldown is decreased by one each
microround (1/10th of around, so accurately a deciround ;->).

The player is only asked for action when his cooldown is 0,
much like other NPCs, wo are only animated in that specific case.

By not setting the cooldown directly, but by using set_round_cooldown(),
one could implement haste/slow states quite easy.