Previously, the SourceInterceptor made the assumption that the QualitySelector button component
is a direct child of the controlBar component. That may not always be true. Video.js allows you
to specify a nested hierarchy of components, and so when plugin users choose to move the
QualitySelector button elsewhere, the plugin does not work properly.
This commit introduces a new event type called QUALITY_REQUESTED to signal when the user is
requesting a quality change. The old QUALITY_SELECTED event is now used to denote when the
plugin actually uses a new quality source. This dichotomy eliminates the need for the
SourceInterceptor to have a reference to the QualitySelector button component.
