Overhaul UI: some minor improvements

This MR contains some small improvements. One is that in the fitting panel a label is shown instead of an empty space to explain that the selected job has no real data associated with it for jobs not ready for fitting.

requested review from @m.puchner

assigned to @m.puchner

approved this merge request

assigned to @t.knopff and unassigned @m.puchner

merged

mentioned in commit 702a4706