Bug 1484 - Cannot one-step extend/contract selections vertically or horizontally to specific TrackSelFunc

Progresses us a little towards closing.  Ctrl-Click on waveform now removes track selection status, not just adds it, and we don't flicker on again subsequently when we drag.  Ctrl-Click won't remove the last remaining selection (by design).  Shift-Click can now reduce us down to one track, rather than the previous behaviour when reducing track selection.

src/TrackPanel.h

@@ -291,6 +291,7 @@ class AUDACITY_DLL_API TrackPanel final : public OverlayPanel {
    // AS: Selection handling
    void SelectTrack(Track *track, bool selected, bool updateLastPicked = true);
    void SelectRangeOfTracks(Track *sTrack, Track *eTrack);
+   size_t GetSelectedTrackCount();
    virtual void HandleSelect(wxMouseEvent & event);
    virtual void SelectionHandleDrag(wxMouseEvent &event, Track *pTrack);