CommentHandling: add support for #| and extend handling #~ comments

[ProbstDJakob]

• renamed auto-comments to extracted-comments and user-comments to translator-comments see GNU PO-Files
• add support for reading and writing "previous" comments (#| ) including previous msgctxt's
• "obsolete" comments of various forms will now be accepted when reading (#~: , #~, , #~. , #~ , #~| )
• write "obsolete" comments with all their details/comments (only "current" msgctxt, msgid, msgid_plural and msgstr were prefixed with #~ )
• when processing (parsing) comments, remove defaulting to translator comments instead call _invalid_pofile with "Unknown comment type" as message

Closes #206
Closes #566

[codecov-io]

Codecov Report

Merging #743 (e8fc9d5) into master (5afe2b2) will increase coverage by 90.94%.
The diff coverage is 93.49%.

@@             Coverage Diff             @@
##           master     #743       +/-   ##
===========================================
+ Coverage        0   90.94%   +90.94%     
===========================================
  Files           0       24       +24     
  Lines           0     4218     +4218     
===========================================
+ Hits            0     3836     +3836     
- Misses          0      382      +382     

Impacted Files	Coverage Δ
babel/messages/frontend.py	87.03% <ø> (ø)
babel/messages/pofile.py	94.25% <92.59%> (ø)
babel/messages/catalog.py	95.87% <100.00%> (ø)
babel/core.py	96.04% <0.00%> (ø)
babel/dates.py	91.60% <0.00%> (ø)
babel/numbers.py	97.70% <0.00%> (ø)
babel/support.py	83.20% <0.00%> (ø)
babel/lists.py	100.00% <0.00%> (ø)
babel/languages.py	100.00% <0.00%> (ø)
babel/localtime/__init__.py	65.00% <0.00%> (ø)
... and 17 more

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