- Have a single engine per-thread with distinct QQmlContext instead
  of one engine per object
- Fully implement custom UI support
- Add new API for storing & retrieving state of JS objects across restarts and
  communicating between UI and execution.
- Whenever possible, load QML files from disk to trigger the disk cache, which will
  enable much faster loading / reloading on the execution thread
- Make sure that engines respect import paths, and change presets to use a qmldir to import JS files,
  so that we do not hit QTBUG-107204