iOS: only force-finish start-up from file engine when on main thread

The authorization dialog that grants the application access to the
assets will only show after returning back from
applicationDidFinishLaunching. Therefore, trying to load an asset from
main before qApp->exec() would normally fail. To remedy that, we added
a path that starts a QEventLoop for a split second to force the
application init process to finish. But this can only work if we
start it from the main thread.

A bug with this is seen in QML FileDialog, since it (clumsy enough)
starts to iterate, in a separate thread, all the files in its currently
set directory upon start-up construction (which should be fixed as
well). The result is that the application gets dead-locked on start-up.

Change-Id: I0047272d53314752903960b33f88b33dc0d7e78d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

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