The loading overlay is meant to signal heavy work to the user, and
should be used whenever the ui feels like  frozen. Therefore it
deactivates all ui-interaction (except key-handling).

There should be only one instance of Loading in the whole application,
which is a sibling of the stack-view. Therefore we pass it into the
activities via the ActivityBase.loading property, as we do e.g. with
audioEffects.

For now we activate it always during activity loading through the
activity-loader. Especially on (slower) Android devices the intial
rendering of the activity-scene takes some time, and so far looked
like a freezing ui to the user.

We might want to activate the loading overlay only on mobile devices,
as a desktop should render its scene pretty fast.