Finalized level export

[Qaomen]

The gist of it:

• Marble maps are now referred to as a "level", to avoid confusion with map/level/project and Map objects.
• Level files have a version. Older levels are automatically converted to newer versions on load.
• Current level version is visible in the editor.
• Level scripts are accessible from client and server. A manager module has helper functions for loading/exporting.
• Complete level validation on load/export. This should be kept up-to-date for any future file changes.
• Level files use MessagePack now. This is not backwards compatible (all levels on GitHub are converted to this new format).
• Finalized gameplay parameters and added export date (for level's unique ID).

Referencing issues Closes #37 Closes #101 Closes #112 Closes #130