parse_one_presentation_attribute() - renamed from parse_attribute_pair()
parse_one_presentation_attribute() - renamed from parse_attribute_pair()

Also, remove the accept_shorthands argument from this function, since
its single caller always passed false (shorthand properties cannot be
used as presentation attributes).

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg