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/TrackPanelResizeHandle.cpp

@@ -317,7 +317,7 @@ UIHandle::Result TrackPanelResizeHandle::Drag
 }
 
 HitTestPreview TrackPanelResizeHandle::Preview
-(const TrackPanelMouseEvent &, const AudacityProject *)
+(const TrackPanelMouseState &, const AudacityProject *)
 {
    return HitPreview(mMode == IsResizingBetweenLinkedTracks);
 }