GtkMenu is initially unowned, which means ref_count is 1 and floating flag is set.
gtk_menu_popup temporarily adds a ref, that will be dropped when the menu is popped
down. So once the menu is popped up, we call g_object_ref_sink() to clear the floating
flag, and g_object_unref() to drop the initial ref. Only the ref from gtk_menu_popup
stay, and the menu will be finalized when it is popped down.

Fixes bug #609567