[GUI] Add prefs for translators

[chennes]

Three changes to make it easier to test new translations:

When creating the original list of locales available to FreeCAD, also scan the supported external directories and add those entries to the list of locales. The additional languages will now appear in the Language preferences drop-down.

Add a hidden preference, BaseApp/General/Preferences/AdditionalLanguageDomainEntries, that will allow translators to add an entirely new language to FreeCAD via user preferences, without requiring modification of the FreeCAD source code. The format is "Language Name 1"="code1";"Language Name 2"="code2";... For example,

<FCText Name="AdditionalLanguageDomainEntries">"Esperanto"="eo";"Polish"="pl";</FCText>

Add a hidden preference, BaseApp/General/Preferences/AdditionalTranslationsDirectory, for an additional translation directory. The specified directory takes priority over all other translation file locations.

Forums discussion: https://forum.freecadweb.org/viewtopic.php?f=8&t=61212

---

• Your pull request is confined strictly to a single module
• Discussed in forum
• Your branch is rebased on latest master
• All FreeCAD unit tests are confirmed to pass by running ./bin/FreeCAD --run-test 0
• All commit messages are well-written
• Your pull request is well written and has a good description
• No tracker ticket

[luzpaz]

Neato!
CC @kaktusus

[yorikvanhavre]

Note: this needs to be adapted as well: https://github.com/FreeCAD/FreeCAD/blob/master/src/Tools/updatecrowdin.py#L308 ...
Maybe this?

    # checking for existing entry
    lastentry = 0
    for i,l in enumerate(cppcode):
        if l.startswith("    d->mapLanguageTopLevelDomain[QT_TR_NOOP("):
            lastentry = i
            if "\""+lncode+"\"" in l:
                print(lnname+" ("+lncode+") already exists in Translator.cpp")
                return

    # find the position to insert
    pos = lastentry + 1

[chennes]

I was envisioning that for actual shipping of translations we'd continue to use the same technique as before, and that this new preference would only be for testing purposes. So that code should still be OK, it will insert any new translations that it finds in CrowdIn into the CPP file, right?

[yorikvanhavre]

Yes... Bu tit was looking for a d->activatedLanguage = "en" line that your PR moved, so now it would probably add new languages at the wrong place. But no worries, let's merge and I'll adapt the script afterwards

[yorikvanhavre]

Done with 455412c