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

CMakeLists.txt

@@ -49,6 +49,10 @@ if(PLOT_BUILD_IMGUI)
   add_subdirectory(adapters/imgui)
 endif()
 
+if(PLOT_BUILD_QT6)
+  add_subdirectory(adapters/qt6)
+endif()
+
 if(PLOT_BUILD_EXAMPLES)
   add_subdirectory(examples)
 endif()