Headword index overhaul

[xiaoyifang]

• replace the headword index with xapian
  current implementation has to deal carefully with CJK characters.
• rewrite the headword dialog
  current situation: headword total number and indexed items are two different numbers GD-ng doesn't find the headwords #1346 Non correct filtered headwords number #1131

[GD-fix]

After a "high priority issue" (with Flatpak paths) was solved, what about this "non high", but, I think, non difficult one?
What is the problem to correct the code, that non correct headwords reading in headwords window before filter applying:

will the same as after filter applying:
?

Thanks in advance.

[GD-fix]

And what is the problem to correct the code, that it will be workable initially, not only after filter applying?

[xiaoyifang]

a compromised solution
#1600