LibGUI: Rename GEventLoop::exit() and GApplication::exit() to quit().

These functions don't exit immediately, but rather on the next iteration
of the event loop.

Since exit() is already used by the standard library, let's call it quit()
instead. That way, saying exit() means the same thing here as anywhere else.

LibGUI/GApplication.cpp

@@ -26,9 +26,9 @@ int GApplication::exec()
     return m_event_loop->exec();
 }
 
-void GApplication::exit(int exit_code)
+void GApplication::quit(int exit_code)
 {
-    m_event_loop->exit(exit_code);
+    m_event_loop->quit(exit_code);
 }
 
 void GApplication::set_menubar(OwnPtr<GMenuBar>&& menubar)