Adding new FNodeEnabler for added+removed

This will make it easier to tell when something has been added or removed from the display list, without having to override HandleAddedToStage() and all that.
Adding GetChildIndex() to FContainer
Additional code for the AddedOrRemoved enabler
Move node depth relative to another node
Adding explicit isMuted to soundmanager
Make sure editor classes are only compiled in editor
Adding RXDivider for creating dividers between editor variables