AVM2: Manually fire late added/addedToStage events on root clips

[kmeisthax]

AS3 display objects always get added and addedToStage events, even the root movie clip. We actually add the root clip before its constructor runs, so it will miss those events. This minor hack fixes that by checking if we just constructed the root clip and, if so, fires those events.

There are no tests because the only tests that revealed this behavior are part of #5876 and test other behaviors

[Herschel]

Could these events be fired manually inside, say, Player after set_root_movie or the first run_frame, or similar?

[kmeisthax]

We can't fire the events in set_root_movie because that's too early: objects are not initialized and thus cannot add event handlers to themselves until the frame construction step.

In #7048, I'm moving all of the AVM2 frame logic into a separate function, and we possibly could send the event there instead, right before we broadcast frameConstructed. The root just inherently knows if it needs to broadcast the event based on whether or not it constructed itself this frame, which is why I put the events there - we'd have to get this information into run_frame_avm2 somehow.

If you're wondering, these are the series of tests that need to get added/addedToStage events on the root in order to pass.

[Herschel]

Thank you!