Add an option to create a note for an undefined tag

[vkbo]

Split from #823:

• It should be possible to set a tooltip for a piece of text (see here), which may be useful to clarify why a tag is not considered valid.
• It should be possible to right-click an undefined tag and select a menu option to create a new note file for it. The file class and therefore root folder can be inferred from the keyword, and the tag value should be used to populate the file with a label, title and the @tag: line.