Handle <a> as container element in SVG

Summary:
<a> is a container element as <g>, so handle both the same.

Allows to index the content of e.g.
https://suchanek.name/programs/powerline/intro/0.svg

Reviewers: Baloo, Frameworks, astippich, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18835