add QCanvas Root and viewport pane in gcompose

app/gcompose/src/QViewportPane.h

@@ -0,0 +1,34 @@
+#ifndef QVIEWPORTPANE_H
+#define QVIEWPORTPANE_H
+
+#include <QWidget>
+
+class QVBoxLayout;
+class QLabel;
+
+class QViewportPane : public QWidget {
+    Q_OBJECT
+public:
+    explicit QViewportPane(QWidget* parent = nullptr);
+    virtual ~QViewportPane();
+
+    void addVtkViewport();
+    void addRootCanvas();
+
+    QWidget* currentViewport() const { return m_viewport; }
+
+private slots:
+    void onCloseRequested();
+    void showContextMenu(const QPoint& pos);
+
+private:
+    void AttemptSplit(Qt::Orientation orientation);
+    void setViewport(QWidget* viewport, const QString& title);
+
+    QVBoxLayout* m_layout;
+    QWidget* m_titleBar;
+    QLabel* m_titleLabel;
+    QWidget* m_viewport;
+};
+
+#endif // QVIEWPORTPANE_H