add QCanvas Root and viewport pane in gcompose

app/gcompose/src/MainPanel.h

@@ -0,0 +1,22 @@
+#ifndef MAINPANEL_H
+#define MAINPANEL_H
+
+#include <QWidget>
+
+class QSplitter;
+class QViewportPane;
+
+class MainPanel : public QWidget {
+    Q_OBJECT
+public:
+    explicit MainPanel(QWidget* parent = nullptr);
+    virtual ~MainPanel();
+
+    QViewportPane* getFirstPane() const { return m_firstPane; }
+
+private:
+    QSplitter* m_rootSplitter;
+    QViewportPane* m_firstPane;
+};
+
+#endif // MAINPANEL_H