[Refactoring] Move path and text positions to `TrainrunSectionViewObject` from `TrainrunSection` model

[emersion]

Currently, path and textPositions are stored in the TrainrunSection model and DTO. These are caches to avoid re-computing the values too often. However, this (1) breaks the DTO abstraction by baking in low-level details about the main editor view and (2) assumes that 1 trainrunsection is always represented with 1 line in the main view (not the case with collapsed nodes).

Move these two to TrainrunSectionViewObject, which is an object specifically tied to the editor main view.