feat(gtk4): GtkGLArea-based plot area and demo

examples/CMakeLists.txt

@@ -30,3 +30,12 @@ if(TARGET plot::qt6)
   target_link_libraries(plot_qt_demo PRIVATE plot::qt6 Threads::Threads)
   plot_apply_warnings(plot_qt_demo)
 endif()
+
+if(TARGET plot::gtk4)
+  # The demo owns the GtkApplication, the window and the header bar; the adapter is
+  # only the plot area, which the window adopts as its content. GTK's include
+  # directories come with plot::gtk4 (see the adapter's CMakeLists).
+  add_executable(plot_gtk_demo gtk_demo.cpp)
+  target_link_libraries(plot_gtk_demo PRIVATE plot::gtk4 Threads::Threads)
+  plot_apply_warnings(plot_gtk_demo)
+endif()