Added add-on creator

[Semphriss]

It is now possible to pack a world or a levelset as an add-on zip archive automatically, form inside the game.

Features:

• Create an add-on automatically from an option in the editor menu; no user input needed.
• Add-on generated as a full, ready-to-ship zip archive in the addons folder.
• Automatically finds the necessary data to fill the .nfo file.
• Versioning system: Packaging the same add-on multiple times increases the version number automatically.

Flaws and shortcomings:

• Author and license for the add-on automatically set to be the same as the level from which the option is called, which might not be consistent/compatible with other licenses if many assets.
• Does not seek for files outside the world's folder; images and other assets will not be packaged if they are not located within levels/<world-id>/.

Closes #717

[tobbi]

#717