Define and use the UIHandle and TrackPanelCell protocols, but...

...no actions reimplemented to them yet.

Later commits will move special cases one at a time from TrackPanel, preserving
all click and drag capabilities at each step.  With a few exceptions, but those
lost abilities are restored in yet later commits.  (Ctrl+Click on the Label
track being one.)

src/TimeTrack.h

@@ -41,6 +41,7 @@ class TimeTrack final : public Track {
 
    virtual ~TimeTrack();
 
+
    Holder Cut( double t0, double t1 ) override;
    Holder Copy( double t0, double t1, bool forClipboard ) const override;
    void Clear(double t0, double t1) override;
@@ -48,6 +49,10 @@ class TimeTrack final : public Track {
    void Silence(double t0, double t1) override;
    void InsertSilence(double t, double len) override;
 
+   HitTestResult HitTest
+      (const TrackPanelMouseEvent &event,
+       const AudacityProject *pProject) override;
+
    // Identifying the type of track
    int GetKind() const override { return Time; }
 
@@ -146,6 +151,10 @@ class TimeTrack final : public Track {
    Track::Holder Duplicate() const override;
 
    friend class TrackFactory;
+
+protected:
+   TrackControls *GetControls() override;
+   TrackVRulerControls *GetVRulerControls() override;
 };