Refactor and extend dataset handling

[ondrej33]

In this PR we present a few improvements to dataset handling and others.

• We now display indices of observations in the dataset table. Users can see the internal order of observations, allowing for the next point.
• We finally support evaluating trajectory (time-series) properties. With that, all existing types of properties are supported.
• Empty observations are now allowed (as the restriction no longer makes sense).
• Dataset variable names can be edited via dialogue. We can thus finally properly create datasets from scratch.
• Results of the consistency check now automatically disappear after the sketch is modified (as they are no longer valid).