Improve HTML extension, add SVG extension.
- Switching file ext shows/hides meta panels as appropriate.
- When editing .html ext, we get live updates.
- Add svg file extension support.
Add image preview support.

- Instead of editing images as text, images are shown as an <img>
  (with a data url containing the b64 data).
- SVG images are special-cased: they are editable.