feat: implement configurable font settings for VTK viewports and GUI elements with persistent preferences.

app/gcompose/src/ViewportPane.h

@@ -8,6 +8,7 @@
 namespace uLib {
     class Object;
     namespace Qt { class PropertyEditor; }
+    namespace Vtk { class Viewport; }
 }
 
 class QSplitter;
@@ -24,6 +25,7 @@ public:
     void addRootCanvas();
 
     QWidget* currentViewport() const { return m_viewport; }
+    uLib::Vtk::Viewport* viewport() const;
     
     /** @brief Update the display properties for the given object. */
     void setObject(uLib::Object* obj);