Bring back support for default parameters

[tomek-he-him]

To make the thing compatible with SVG Parameters, I originally included a compatible way to declare default parameters. You could declare the value through a <ref> element.

But at the moment default parameters costed a significant amount of tests, logic and bugfixes in the proof-of-concept library. Since I don’t need them yet, I’d rather keep them out for v1.0.

We can always re-introduce them in v1.1 or later without that being a breaking change.

[tomek-he-him]

Here’s where I started getting rid of the defaults: 5cc0455.