Support simple workflow with multiple exports

[fsteeg]

Came up in #106 (comment).

It should be possible to export-edit-import workflows multiple times without moving or renaming files or editing file names referenced in the files. Currently, exported files are downloaded immediately, appending (1), (2) etc. to the file names, causing the workflows to error after import, if the files are not renamed before importing.

This could be via 1) a way to select the download location when exporting, and saving to a different folder for each export or 2) generating unique file names for the exported files (not sure it 2) would help though, since currently, if I reference e.g. playground(1).fix in a playground(1).flux I can't run the imported workflow as well).

Or maybe 3), somewhat related, not sure if we discussed this before, could it make sense to bundle the files as a ZIP, expecting a certain internal structure (playground.data, playground.fix, playground.flux), but making the name of the ZIP file itself, which is exported and imported, arbitrary?