Add a Tags View

[vkbo]

It can be pretty tricky to rename or manage tags for a large, active project. I propose to add a Tags View, similar to the Novel View where the tags are listed by category. Potentially, also with all references listed under a collapsible menu.

It will partially replace, or at lest complement, the References view under the Viewer. It could potentially mean that at least the References tab can be dropped, but the other tag tabs are still useful as they contain more information than the Tags View would have room for.

As a bonus, an additional category can be viewed in the Tags View where all undefined references are listed. This would make it much easier to find them.

[awqk]

Something else I noticed:
If there are multiple tags in a project note referenced from the same document, only one reference is shown in the References panel.
Edit:
Already fixed: References always point to the beginning of the file, and not to the spot where they appear (#1987)

[vkbo]

The references list is meant to be a list of documents that reference current note. It is filtered against duplicates, so this is as intended.