[Refactoring] Replace `TrainrunSectionTabComponent.selectedTrainrunSection` with an array

[emersion]

TrainrunSectionTabComponent might deal with multiple trainrun sections for non-stop or collapsed nodes. Currently, we need to be careful everywhere and use helpers such as getNextStopLeftNode()/getNextStopRightNode()/getLeftRightSections() where appropriate.

Replace TrainrunSectionTabComponent.selectedTrainrunSection with an array with the full trainrun section chain. This makes it so all users need to be explicit about what they want (first, last section, or a computation over the full list), instead of defaulting to the first section.