Repository re-organization, cleanup and add-on management, using Git history

[provigz]

Note: This PR shouldn't be squash-merged, because the commit hashes some add-ons rely on to be downloaded, may be lost.

This pull request makes major changes to how add-ons are organized and maintained in this repository. It aims to:

• Help improve orientation for new users, who are looking to add/modify a SuperTux add-on.
• Clean up many unneeded files, or move them to an appropriate location, where they couldn't be mistaken as needed.
• Allow add-ons to be downloaded, using git commit hashes. This is very beneficial for backwards compatibility with add-ons for different SuperTux versions, and it doesn't require storing many version files of the same add-on.
• Add a new index-0_7.nfo file, which will store updates for add-ons for the future SuperTux 0.7.x releases (like the contrib-type property addition also included here).

Because of the third thing this PR aims to do, add-ons would need to be added/updated slightly differently. This is described in detail in the updated README this PR also includes. The benefit of this is that storing add-ons should be way cleaner.

Included here are also miscellaneous other additions to add-ons, unrelated to add-on re-organization:

• Reversion to old add-on versions for SuperTux versions 0.5.x, 0.4.0 and 0.3.5, so they can download their supported old add-on versions. Originally considered to be added in Revert to old addon versions to fix addon errors in previous SuperTux versions #35.
• Addition of the contrib-type property, previously mentioned. Originally considered to be added in Update worldmaps contrib-type #32. This addition is only available in the index-0_7.nfo file (for future 0.7.x versions).
• Update to the "Island of Moob" worldmap, which also includes the contrib-type property addition. Originally considered to be updated in Fix backgrounds + misc in Island of Moob #36.
• Addition of the new add-on "The Crystal Catacombs" for 0.6.x and 0.7.x versions, also including the contrib-type property addition.

Closes #14.