Split off activate to activate and show callbacks, and split off
deactivate to deactivate and hide callbacks.  Sources didn't previously
have a means to know whether it was actually being displayed in the main
view or just happened to be visible somewhere.  Now, for things like
transition sources, they have a means of knowing when they have actually
been "activated" so they can initiate their sequence.

A source is now only considered "active" when it's being displayed by
the main view.  When a source is shown in the main view, the activate
callback/signal is triggered.  When it's no longer being displayed by
the main view, deactivate callback/signal is triggered.

When a source is just generally visible to see by any view, the show
callback/signal is triggered.  If it's no longer visible by any views,
then the hide callback/signal is triggered.

Presentation volume will now only be active when a source is active in
the main view rather than also in auxilary views.

Also fix a potential bug where parents wouldn't properly increment or
decrement all the activation references of a child source when a child
was added or removed.