Support for publishing UICatalog as single file

[tznind]

I was experimenting with <PublishSingleFile>true</PublishSingleFile> and UICatalog. I have discovered that it more or less works. Only barrier I had was IL3000

Solution is here just to fallback on System.AppContext.BaseDirectory.

This PR does not itself change to single file, it just strengthens the existing code so that if it were run under such build options then it would not crash.

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