feat(qt6): QOpenGLWidget-based plot widget and demo

examples/CMakeLists.txt

@@ -21,3 +21,12 @@ if(TARGET plot::imgui)
     Threads::Threads)
   plot_apply_warnings(plot_imgui_demo)
 endif()
+
+if(TARGET plot::qt6)
+  # The demo owns the QApplication, the QMainWindow and the toolbar; the adapter is
+  # only the widget, which the window adopts as its central widget. No macro
+  # processing is needed here: the demo declares no Qt class of its own.
+  add_executable(plot_qt_demo qt_demo.cpp)
+  target_link_libraries(plot_qt_demo PRIVATE plot::qt6 Threads::Threads)
+  plot_apply_warnings(plot_qt_demo)
+endif()