Add an example desktop file, and install it into
/usr/share/applications.

Run "autoreconf --force --install" to create the build system, then run
"./configure" to check for necessary dependencies, such as make.
Finally, run "make install" to install (into the default prefix).
Examine the output from "./configure --help" to learn how to change the
prefix and other installation settings.

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