TrackPanelCell hit tests can return multiple results...

.. though only the first is used yet

src/TrackPanelResizerCell.h

@@ -24,7 +24,7 @@ public:
    explicit
    TrackPanelResizerCell( std::shared_ptr<Track> pTrack );
 
-   UIHandlePtr HitTest
+   std::vector<UIHandlePtr> HitTest
       (const TrackPanelMouseState &, const AudacityProject *) override;
 
    std::shared_ptr<Track> FindTrack() override { return mpTrack.lock(); };