Add UI elements for time-based topology selection

[gandreadis]

A simulation can have a topology that changes over time (at certain ticks). Currently, only the last version of that topology can be edited, but the user should have a way of selecting which version to edit (and where to branch off from the current topology path). Due to the difficulty of rendering this in a user-friendly way, this will probably have to be a timeline of some sorts.

Figuring out if there are already other software modules doing this in an acceptable way is important, as rendering of these paths can become quite complex.