Add two commands to move the cursor/selection to next/prev label

The two commands are "selection to next label" and "selection to previous label".
They have default shortcuts alt+right and alt+left.

A label track does not have to be the focus. If there is a single label track in the project, that it used. If there is more than one label track, then the first label track, if any, starting at the focused track is used.

If the commands are used during playback of the project, playback continues from the new cursor/selection.

The commands provide feedback to screen readers: the name of the label, and position in the form of "i of n".

src/TrackPanelAx.h

@@ -41,6 +41,8 @@ public:
    // Called to signal changes to a track
    void Updated();
 
+   void MessageForScreenReader(const wxString& message);
+
 #if wxUSE_ACCESSIBILITY
    // Retrieves the address of an IDispatch interface for the specified child.
    // All objects must support this property.
@@ -108,6 +110,10 @@ private:
 
    TrackPanel *mTrackPanel;
    Track *mFocusedTrack;
+
+   wxString mMessage;
+   bool mTrackName;
+   int mMessageCount;
 };
 
 #endif // __AUDACITY_TRACK_PANEL_ACCESSIBILITY__