Rename native SVG animation tags

(For merging into project branch smil-in-js)

The following element tags supporting native SVG animation:-
- animate
- animateMotion
- animateTransform
- mpath
- set

have been renamed to:-
- nativeAnimate
- nativeAnimateMotion
- nativeAnimateTransform
- nativeMPath
- nativeSet

The base class SVGAnimationElement has been renamed to SVGNativeAnimationElement, and the IDL and C++ classes for the elements have similarly been renamed.

This change leaves the original tag names free for use with the upcoming JavaScript Polyfill implementation of SVG animations.