Add purpose for crystals in Icy Island bonus level

[Wuzzy2]

SuperTux version: 0.6.1 beta 1 System information: GNU/Linux

Feature request

There is a bonus level in Icy Island. There are 4 hidden crystals. There is also an info block that says there are 4 hidden crystals, but that they do not have any purpose yet, but that they might have a purpose in future versions.

Therefore, I suggest to give them a purpose. I have two mutually-exclusive suggestions, actually.

Short-term suggestion

Display in the HUD how many crystals you already got.

Add a special area in the level that has a wooden door, and it opens when all 4 crystals have been collected.

The special area could contain a large amount of coins, maybe even tux dolls or other goodies.

Long-term suggestion

When all 4 crystals have been gotten, unlock access to another bonus level.

Other suggestions

Feel free to post them here.