GTK's Main Loop

After initializing the library and creating its windows (we'll see this later),
we should pass control to GTK's main loop function, as follows:

 /* All GTK applications must have a gtk_main(). Control
  * ends here and waits for an event to occur (like a
  * key press or mouse event).
  */
 gtk_main();

This function will only return when the application tells GTK to exit, by using the following code in one of the callbacks:

 /*
  * mark that we should exit as soon as control 
  * returns to the gtk_main() function.
  */
 gtk_main_quit();