Fix logic error in layout translation extraction

lyx_pot.py treats the combination IsPredefined == true and UsesFloatPkg == true
incorrectly: In this case it does not extract the float name, although it needs
to be extracted. This was no problem for LyX 2.0.0, because no layout contained
this combination fo flags. However, the current version of achemso.layout uses
it for several styles, e.g. "List of Schemes".
You only see the bug if you remove lib/layouttranslations before updating it,
otherwise it would be covered by the old translations which are kept. The fix
make use of the fact that IsPredefined == false and UsesFloatPkg == false is
not supported by LyX to simplify the logic.
Remove context also for translated strings

This is consistent with the treatment in LyX itself (cleanTranslation() is
also called on the translated strings), and some translators leave the context
in the translated msgstr (e.g. [[List of Listings]]), although that is not
recommended. Without this, lib/layouttranslations would contain wrong
translations.
UserGuide.lyx: improve a writing style
Add new layout translations to be reviewed

Now lib/layouttranslations.review contains all translations which have been
changed since the 2.0.0 release except for the ones where the commit log
mentioned that they were reviewed.
Transfer layout translations to fi.po

These translations were only in lib/layouttranslations, and they were already
reviewed, so they are good for the GUI as well.
Transfer layout translations to nl.po

These translations were only in lib/layouttranslations. I don't know if they
were reviewed, but I understand enough dutch to be sure that they are not
completely wrong, so they are good enough for the GUI.
installer: update dictionary list

- there is now a Turkmen spell checker
- there is now a Galician thesaurus available
Enable environment-split for standard classes (#9023)