Generating all images for a gallery view can take
long. It might not be obvious when the computation
is done, because there is no progress indicator.
Fixed by adding a progress bar.
You might want to abort the computation. E.g.
because you already found what you were looking
for or you want to change the parameters.
This can now be done by clicking 'Abort'.
Note, it does not abort the current computation.
We do not know which fields exist at compile time.
But it is a great help to have some pre-selected
fields in groupBy.
Solved by adding a configuration option.
