timeline: Move some docstrings to be inside the methods they refer to
utils/timeline: Provide context to a comment with a protected historical status
timeline: Clarify the scrollToPosition code and add programming safeguards

If out-of-bounds values are provided, warnings are emitted to aid debugging
timeline: Fix the "Zoom fit" and automatic scrolling behavior for clip insertion

- On drag and drop, call scrollToPosition with an X position, not nanoseconds;
  this prevents the timeline from scrolling to the end everytime.

- While inserting (multiple) clips, cache the value of zoom_fitted;
  this prevents repeated insertions from deactivating the fitted zoom

- Add a margin on the left of clips we scroll to during a drag & drop insert
- Simplify scrollToPlayhead's call to scrollToPosition
- Add more debugging statements