Component example showing composition of components (#150)

This approach can allow for UI sections that are more modular and reusable

doc/example_list.md

@@ -40,6 +40,7 @@
 @example ./examples/component/menu.cpp
 @example ./examples/component/menu_style.cpp
 @example ./examples/component/radiobox_in_frame.cpp
+@example ./examples/component/composition.cpp
 @example ./examples/component/button.cpp
 @example ./examples/component/toggle.cpp
 @example ./examples/component/modal_dialog.cpp