Fix issue #516
Fix issue #516

- Don't show more than once "Do you want to delete old data" when
loading multiple files.
- Correctly clean all the data, including _loaded_datafiles

facontidavide/PlotJuggler,facontidavide/PlotJuggler,facontidavide/PlotJuggler,facontidavide/PlotJuggler,facontidavide/PlotJuggler