Kexi/Migration

TODOs

Porting to non-modal assistants

What is involved in porting to the new gui? <jstaniek>: We sit and design the given assistant and I am helping with using the API and possibly extending/fixing it. The difference is that many imports can be started concurrently so some extra checking may be needed but this is a side note. Also e.g. user could try to edit/remove a source table during export. So this is fun. But after the change the GUI would be a lot more natural I think.