Updated to use Qt6; Created initial structure of the viewer with OpenGL widget

src/qtviewer/IfcViewerWidget.h

@@ -0,0 +1,23 @@
+#ifndef IFCVIEWERWIDGET_H
+#define IFCVIEWERWIDGET_H
+
+#include <QOpenGLWidget>
+#include <QOpenGLFunctions>
+#include <QOpenGLContext>
+#include <QMatrix4x4>
+
+class IfcViewerWidget : public QOpenGLWidget
+{
+public:
+    IfcViewerWidget(QWidget *parent = nullptr);
+
+protected:
+    void initializeGL() override; 
+    void resizeGL(int w, int h) override;
+    void paintGL() override;
+
+private:
+    QMatrix4x4 m_projection;
+};
+
+#endif // IFCVIEWERWIDGET_H