Fixes #3698. ResourceManager GetResourceSet doesn't fallback to default for no translated keys.

[BDisp]

Fixes

• Fixes ResourceManager GetResourceSet doesn't fallback to default for no translated keys. #3698

Proposed Changes/Todos

• Add internal ResourceManagerWrapper class.
• Add GlobalResources class helper which will handle the ResourceManagerWrapper. I advice to use the static GlobalResources to deal with all Strings resources.
• Add a new GetResourceSet method with a adicional parameter filter Func<DictionaryEntry, bool>?.
• Add ResourceManagerTests file with unit tests.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

Nice. See formatting comment.