Cleanup subtitle and preview handling with portals

src/App.test.tsx

src/api/ApiClientContext.ts

src/routes/ContentRoute.tsx

src/routes/main/MainPage.tsx

src/routes/player/Player.tsx

src/routes/player/PreviewBar.tsx

src/routes/player/SeekBar.tsx

src/routes/player/subtitles/SubtitleRenderer.tsx

src/routes/player/subtitles/TtmlRenderer.tsx

src/routes/player/video/DashVideoElement.tsx

src/routes/player/video/PlayerApi.ts

src/routes/player/video/RawVideoElement.tsx

src/routes/player/video/Track.tsx

src/routes/player/video/VideoContext.tsx

src/routes/player/video/VideoElement.tsx

src/util/media/useAudioTracks.ts

src/util/media/useDuration.ts

src/util/media/usePosition.ts

src/util/ttml/html.js

src/util/ttml/ismc.ts