Implement an XMLNode class inherited by Tag and the other XML node types

[MewPurPur]

XMLNode ---> UnknownXMLNode, CommentXMLNode, CDataXMLNode, TextXMLNode, Tag

This would allow to preserve these things in SVGs. None of them have an effect on rendering, although text nodes can be relevant inside text elements (which aren't supported yet in ThorVG, but still, importing an SVG in our tool shouldn't corrupt it). These would all have their own tag editors in the inspector that allow to edit them.

Prerequisite (or a full solution?) to #548.