Support mass export and import

[Glodenox]

It would be nice to able to export more than one layer at a time. Ideally, this would be in a zip file (see https://github.com/Touffy/client-zip maybe), but otherwise it could be a huge JSON as well.

Preferably, make it possible for people to select which layers to export instead of just all of them. That would allow people to share projects with multiple images amongst each other.