Exporting / sharing scene

It would be neat to have a way to export a scene for sharing it with others to let them help with rendering.

This could be done by copying/compressing relevant files into a ZIP while excluding irrelevant files like snapshots and backups. This should also include embedded custom textures as described in #1652.

Exported scenes can probably not resolve the world from which they were created (at least when copied to other computers), in which case the world map tab as well as the reload chunk buttons should probably be disabled.

This feature could also be used to archive old renders to save space. In this case the user should have to select which snapshots should be included.

No files or tests are named. Begin by reviewing the embedded custom texture handling described in #1652, then define the export and archive contents, unavailable world-map and reload controls, and selectable snapshot behavior.

Written by the indexing model from the issue text.