Application may be long to start (e.g. a Docker app may have to download images first,
and even without the downloading, the starting could be long), which may lead to UI
blocking or bridge time out.
To prevent that, `start` is now returning immediately, and 2 new signals are used to
indicate when the application is started, of if something wrong happened.
`start` now returns initial app data, including exposed data without the computed exposed
data. The computed data must be retrieved after the app has been started.
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