infobar: deprecate broken GtkInfoBar style properties

infobar: deprecate broken GtkInfoBar style properties

The way we use these style properties to set regular properties on
containers accessible from the public API is really just broken, and
could lead to undefined values for the spacing and border-width
container properties (since they could be set from public API and then
changed from under in a style_update handler from GTK).

Take this as an occasion to deprecate these style properties, which do
not make a lot of sense anyway, now that GtkInfoBar supports regular CSS
padding and border.