Because the result will be a float anyway get rid of the int parsing.
Also the grammar of SVG numbers matches the double parser grammar except
it can't have a sign but that should have been checked by the caller.
This is all still quite ad-hoc. Eventually these will both need to
support units (like with CSS Lengths) but for now we can continue only
using numbers.
I've chosen the name `AttributeParser` since it parses data from
attributes. Rather than duplicate the parsing of numbers and other
basic types, let's make use of this existing parsing code for parsing
the data for `<line>`, `<polyline>`, etc.
