Preserve XML markup in imported full-text documents

[davidzbiral]

Some editions of texts come as an XML (or TEI-XML) document. Allow the import of such documents, and make the following changes to Annotator:

• Preserve all of them and their attributes.
• Only show these in xml mode. In highlight mode and text edit mode, hide them, only show text.
• Ensure the tags cannot be corrupted by either anchor adding (i.e. anchors must not be added into XML tags) or plain text editing in text edit mode - only allow their changes in XML mode.
• Our UUID anchors can go across start and end tag, thereby creating invalid XML (because of overlapping markup: XML start tag 1 is before XML start tag 2, but xml end tag 1 is after XML end tag 2, which is invalid in XML). But while allowing this situation, do not contribute to it - i.e. if the UUID anchor is adjacent to another xml tag (in the XML view), make as little crosses of markup as possible when adding our UUID anchor (related to Anchor overlap prevention  #2403, just putting the original XML into the picture).

If needed, in full-text import and the rendering of anchors, ensure the recognition of InkVisitor database UUIDs which should pair up, telling them apart from other <>-delimited XML markup.