The PR may seem large, but the main changes are simple:
- Introduce a `DomManager` that aims to take over all DOM responsibilities from `FlutterViewEmbedder`.
- Update all references to `flutterViewEmbedder.*domElement*` to `domManager.*domElement*`.
- Describe the general DOM structure of a Flutter View in a doc comment.
Next steps (in future PRs):
- Move all DOM manipulation methods out of `FlutterViewEmbedder` into `DomManager`.
- Move DOM creation logic out of `FlutterViewEmbedder` into `DomManager`.
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