Bug 1015412 - [FMRadio] Implement new startup loading events

In the case of this app, the buttons/outlines of navigation are present in index.html. However, the radio dialer bar isn't rendered until this point (hash marks and numbers).

Also if the app would cease execution at the point when `moz-app-content-interactive` is emitted (meaning the favorites list isn't loaded), the app would still be usable (just without the favorites list).