Create a button, and link it to the "app.hello-world" action, so that
clicking it does the same as selecting the menu item or pressing the
keyboard shortcut for that action.

Create a menu model, and add menu items into the model. Link the menu
items to the "hello-world" and "quit" actions, which were added to the
application. Set the menu to be the application menu.

Create two actions, "hello-world" and "quit", and add them to the
application (which is an implementation of GActionMap). Connect the
"activate" signal of the actions to handlers. Add an accelerator for
each action, so that they can be triggered with a keyboard shortcut.

Use GtkApplication to manage the application lifecycle, and quit the
application when the window is closed.

The appliction shows a window and runs forever. Closing the window does
not stop the application, so it must be killed externally.