Added static Starling.all property to get all Starling instances

Similar to `Starling.current`, but it gets all Starling instances. An instance is added in its constructor and removed when its `dispose()` function is called.

## Use Cases
1) I have a reference to a `starling.display.Stage`. It would be nice if I could somehow get the `starling.core.Starling` instance that owns it. For instance, I might want to know the `nativeStage` that is associated with this Starling stage so that I can position something properly above Starling in an AIR `NativeWindow`. Looping through the vector returned by the static `Starling.all` property would allow me to find the right Starling instance pretty easily.

2) An AIR `NativeWindow` has a `stage` property, which makes it easy to access the native display list of the active window with `NativeApplication.activeWindow.stage`. However, there's no way to get a Starling stage for an arbitrary `NativeWindow`. Similar to the previous use case, I can loop through the static `Starling.all` property and check if `NativeApplication.activeWindow.stage` matches a Starling instance's `nativeStage` property.