Warn when a property is set on a GtkPaned with no children

If a {resize,shrink}_{start,end}_child properties are set on a gtkpaned and then a child is added, those properties will be reset. This can be annoying as a dev if you don't know why this is happening. So this commit adds a warning that tells the dev if a property is set on a paned with no children, that tells them that the prop will be reset when a child is later added and suggest setting the prop after the child has been added.

Related to #5500