Remove existing "embedded instance" options:
- `embedded-sr`
- `embedded-sr-type`
- `embedded-instance-parameters`

Instead introduce `one-time-embedded-instance` option:
- Only taken into account when Theia is instantiated the first time.
- Content should be a JSON object with:
  - 'software-url': string of the software URL
  - 'software-type': (optional) string of the software type
  - 'instance-parameters': (optional) JSON object

If the option is non-empty:
- Theia will attempt once to:
  - Parse the JSON content
  - Create a supply/request script
  - Call the script
- A promise will check whether the script was successfully called.

If the promise fails forever, e.g. because of ill-formed JSON, emptying
the option will remove the promise. Editing the option after the first
time will have no effect, whether the promise succeded or not.