creation of datasets / intervals should trigger a modal dialog

[behrisch]

creating new data sets would give better feedback if it were a button opening a modal dialog asking for the name / the interval boundaries. A checkbox is probably not what the user expects here. We could think about something similar for creating vehicle types

[namdre]

The following might already solve most of the problems:

• if no dataset is currently loaded, the frame should be in the "create dataset" mode by default
• if the current dataset has no interval, the frame should be in the "create interval" mode by default