SVGNodeContainer methods should be chainable

[meyfa]

To make constructing/editing larger node tree parts easier, SVGNodeContainer::addChild() and SVGNodeContainer::removeChild() should return the node container instance they were called on. This would allow for method chaining exactly like the other node methods already offer.

The return values, which are rarely (if ever) used anyway, could rather easily be replaced with some code on the client's side (a simple for loop over all children, checking whether the subject is among them, would suffice).