uiMainStep does not work with gtk backend

it always returns false. Some investigation shows that this is because the function gtk_main_iteration_do(), which is used to implement this, needs a running main loop, and that is only created when calling gtk_main() (see https://git.gnome.org/browse/gtk+/tree/gtk/gtkmain.c?h=3.18.9). So i see no easy way to fix this... maybe an idle callback would be a prudent replacement, that should be available on all platforms...