add QCanvas Root and viewport pane in gcompose

app/gcompose/src/MainWindow.h

@@ -4,9 +4,10 @@
 #include <QMainWindow>
 #include <QVTKOpenGLNativeWidget.h>
 
+class MainPanel;
+
 namespace uLib {
 namespace Vtk {
-class QViewport;
 }
 }
 
@@ -16,10 +17,10 @@ public:
     MainWindow(QWidget* parent = nullptr);
     virtual ~MainWindow();
 
-    uLib::Vtk::QViewport* getViewport() { return m_viewport; }
+    MainPanel* getPanel() { return m_panel; }
 
 private:
-    uLib::Vtk::QViewport* m_viewport;
+    MainPanel* m_panel;
 };
 
 #endif