v0.6 update

Some improvements are made and newly discovered bugs are fixed:

1. Adaptive difficulty system improved: the game will become easier gradually when the player repeatedly failed at a checkpoint, this includes but not limited to reducing the damage received and faster mana regeneration.
2. The checkpoint trigger after escorting the passengers back to the platform is easier to be triggered.
3. Skill "Shield" takes fewer energy and regenerates health faster.
4. Dash is no longer available if there's no mana left.

Known issues:

1. Sticky keys prompt may pop up in Windows while continuously dashing. I will try to find a way to stop it from popping up. Meanwhile the player may try not to dash too frequently. (sorry)