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/GWindow.cpp

@@ -46,7 +46,7 @@ void GWindow::close()
     // FIXME: If we exit the event loop, we're never gonna deal with the delete_later request!
     //        This will become relevant once we support nested event loops.
     if (should_exit_app_on_close())
-        GEventLoop::main().exit(0);
+        GEventLoop::main().quit(0);
     delete_later();
 }