German translation: Initial translation (draft)

[janezd]

Add German translation, one German-translation-related fix in Slovenian translation.

Possible problems (among others):

Bad translations, e.g. bad wording, a different word used in translation.
Incorrect translations of code in f-strings, e.g. translated variable names, incorrect calls of plde.
"Strings" that must not be translated. These were marked as "must not be translated" in the Slovenian translation, which is used as template, but I could have missed some. E.g. in open(..., error="strict"), I did't mark "strict" as non-translatable, and in the German translation it was translated to "strikt".
The first problem causes embarrasment, the latter two cause runtime errors.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.09%. Comparing base (e19999d) to head (f56f8d1).

Files with missing lines	Patch %	Lines
orangecanvas/localization/de.py	0.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage   76.14%   76.09%   -0.05%     
==========================================
  Files         102      103       +1     
  Lines       21321    21335      +14     
==========================================
  Hits        16234    16234              
- Misses       5087     5101      +14     

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