Fix loading states of nested overlays

[peppy]

Ranking and dashboard overlay have this loadDisplay logic which loads a sub component. This sub component is flowing (auto size y) content which usually has a load state. Currently there is no sane way to show a "loading" overlay due to the fact that the vertical size could be zero or changing.

I propose that FullScreenOverlay (or a derived class) exposes a bindable loading state (or something equivalent) to allow these sub components to set the state and have the parent manage the loading display. Both of the above overlays mentioned already have a loading display which could be used for this purpose.

[EVAST9919]

So basically FullScreenOverlay should have loadDisplay and a Bindable so each sub-component could use it instead of each sub-component having it's own loadDisplay ?

[peppy]

Something like that. Maybe utilising OngoingOperationTracker that was introduced in multiplayer recently.