Implement support for <text> element

[rlitv]

It would be nice to have elements in parsed DOM even if they are not supported by svgren, so their presence can at least be checked after parsing.

[igagis]

The <text> element added to svgdom version 0.3.18, the svgren was also rebuilt because of binary incompatibility, so the actual version is 0.4.50.
For now it only inserts the <text> element to the DOM tree, but it does not parse any specific attributes of the element. But this should be enough to detect that SVG contains <text> elements. It should be pretty easy to create a custom Visitor for that, but if you have questions, please ask.