core, add loading overlay and activate it while loading activities
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.
Showing with 374 additions and 2 deletions