Gui: Cleanup Qt OpenGL usings in QtOpenGL.h.

Previously the code defined compatiblity usings in `QtOpenGL.h` header,
which I think was added for backwards compatiblity with previous Qt
OpenGL widgets.

As far as I can tell, this is not necessary anymore, and can be cleaned
up.

src/Gui/Flag.cpp

@@ -43,7 +43,7 @@ using namespace Gui;
 //       Embed complete widgets
 
 Flag::Flag(QWidget* parent)
-  : QtGLWidget(parent), coord(0.0f, 0.0f, 0.0f)
+  : QOpenGLWidget(parent), coord(0.0f, 0.0f, 0.0f)
 {
     this->setFixedHeight(20);
     setAutoFillBackground(true);
@@ -124,7 +124,7 @@ void Flag::setText(const QString& t)
 
 void Flag::resizeEvent(QResizeEvent* e)
 {
-    QtGLWidget::resizeEvent(e);
+    QOpenGLWidget::resizeEvent(e);
 }
 
 void Flag::mouseMoveEvent(QMouseEvent *e)