`@remotion/studio`: Consolidate timeline `showInTimeline` filtering

Replaces the overlapping `isTrackHidden` helper and redundant
`showInTimeline` checks with a single `shouldShowTrackInTimeline`
predicate, so the rules for whether a track appears in the timeline
list live in one self-documenting place.

Closes #7471

Co-authored-by: Cursor <cursoragent@cursor.com>