Previously, we only supported very basic numbers and a single level of
text positioning support in the `x`, `y`, `dx` and `dy` attributes in
`<text>` and `<tspan>` SVG elements.
This improves our support for them in the following ways:
* Any `length-percentage` or `number` type value is accepted;
* Nested `<text>` and `<tspan>` use the 'current text position'
concept to determine where the next text run should go;
* We expose the attributes' values through the API.
Though we still do not support:
* Applying the `rotate` attribute;
* Applying transformations on a per-character basis.
* Proper horizontal and vertical glyph advancing (we just use the path
bounding box for now).
Before this change, we were going through the chain of base classes for
each IDL interface object and having them set the prototype to their
prototype.
Instead of doing that, reorder things so that we set the right prototype
immediately in Foo::initialize(), and then don't bother in all the base
class overrides.
This knocks off a ~1% profile item on Speedometer 3.
We currently have 2 virtual methods to inform DOM::Element subclasses
when an attribute has changed, one of which is spec-compliant. This
patch removes the non-compliant variant.
