Improve custom export formats

[ghost]

Is your feature request related to a problem? Please describe.

The format creation process does not allow the inclusion of image ids and has no way of customizing the separator.

Describe the solution you'd like

The format creator/editor should include %%imageid that returns the id of the image. It should also allow specifying how to separate multiple values. Right now it seems to use line breaks which is hardcoded.

Describe alternatives you've considered

One could just create an intermediary format that is processed after downloading the export, but then there's no point in specifying a custom export format.

[jaagut]

It should also be possible, to include the imageset ID in the name of the export file.