More conservative version of IApplication

[tznind]

More conservative version of #3872 that seeks only to abstract the initial set of methods the user actually interacts with in a basic Terminal.Gui application.

• Init
• Run
• Shutdown

Fixes

• Fixes #_____

Proposed Changes/Todos

• Todo 1

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

[tznind]

Closing this because it is now fully implemented in #3837 (as far as is needed for sensible common functionality).