Tagger: Introduce helper class for tags.

Take the opportunity to clean up some API issues.
Internal: PaperList passes fields around in preference to fieldIds.
PaperList::showHeader defaults to don't show header.
Consolidate unused field and list types.
Paper list: Each field gets a unique name.
Internal: Add helper Qobject class for quiet objects.

Constant use of "defval()" to avoid warnings on unset properties grows
tiresome.
Internal: Use Qobject to simplify information export from PaperList.
Internal: Refactor paper columns so they render themselves.
Internal: Refactor paper columns so they sort themselves.
Internal: Remove $paperListFolds global in favor of per-field information.
Internal: List contents are field names.
Mark a TODO.
Internal: Remove field IDs from consideration.

Push field objects through sorting and special sorting. Also allow
sorting by a field that's not displayed.