implement GUI way to compare sessions

[gdementen]

Either

• via a specific menu (which would have the advantage to be able to compare files directly -- potentially without loading them entirely)

or

• after view several sessions at the same time #10 is merged, via loading several sessions, selecting several and right-click->compare like in implement GUI way to compare arrays #227.

Second option seems very consistent with #227, and is more general (it can compare computed/in-memory sessions) but selecting files on disk directly is one less step and probably closer to what our users do most of the time, so maybe implementing both would make sense.