Matinee: Animation keyframes cannot be added after opening a new asset editor in the same window

When opening a new asset editor in the same window as matinee, the 'Add Key' button for animation tracks will no longer function. This is due to FSlateApplication::ActiveTopLevelWindow being reset and not being properly set back to the active window.

1. Open content browser and matinee tabs and make sure they're in the same top level window.
2. Open any asset editor from the content browser (as long as it opens it in the same top level window as the content browser and matinee - I'm not sure how to guarantee this).
3. Click back to the matinee tab and try to add a keyframe to an animation track.