Revert to old addon versions to fix addon errors in previous SuperTux versions

[provigz]

Makes changes to the main indexes of SuperTux versions 0.3.5, 0.4.0 and 0.5.x to fix errors, occuring in almost all addons. Reverts to old versions of them.

Also:

• Makes a copy of the addon "The Frozen Mountains", so it has both an old version ("v1") and a new one ("v2").
• Removes the addon "Serano's Islands" from the 0.5.x index, because it doesn't work on those versions.

[serano01]

Yeah, sorry about that. I didn't expect that someone still uses the old 0.5-versions, so I only changed it for the then actual version.

[provigz]

Moved to #37.