Rejig EditLabels Dialog.

- Made it more like the Edit Chains Dialog. (Buttons on right).
- Now using ShuttleGui somewhat.
- Gave it a Help button.

src/LabelDialog.h

@@ -27,6 +27,7 @@ class RowData;
 class EmptyLabelRenderer;
 class LabelTrack;
 class ViewInfo;
+class ShuttleGui;
 
 typedef std::vector<RowData> RowDataArray;
 
@@ -54,6 +55,12 @@ class LabelDialog final : public wxDialogWrapper
 
  private:
 
+   void Populate();
+   void PopulateOrExchange( ShuttleGui & S );
+   void PopulateLabels();
+   virtual void OnHelp(wxCommandEvent & event);
+   virtual wxString GetHelpPageName() {return "Labels_Editor";};
+
    bool TransferDataToWindow() override;
    bool TransferDataFromWindow() override;
    bool Validate();