Distinguish TrackPanelMouseState from TrackPanelMouseEvent...

... Let cell hit tests, and handle preview, know states only, not transitions.

Cell hit tests are passed a mouse state that does not always match the current,
but anticipates the button click to come; usually left, but if the Control
[sic] key on Mac is down, then right.

Thus, pressing and releasing Mac Control in multi-tool switches in and out of
the magnifier cursor.

src/TimeTrack.h

@@ -50,7 +50,7 @@ class TimeTrack final : public Track {
    void InsertSilence(double t, double len) override;
 
    HitTestResult DetailedHitTest
-      (const TrackPanelMouseEvent &event,
+      (const TrackPanelMouseState &state,
        const AudacityProject *pProject, int currentTool, bool bMultiTool)
       override;