Fixes #2921 - MainLoop refactoring

[tig]

Fixes #2921

IMainLoopDriver and ConsoleDriver were originally designed to be decoupled by Miguel to enable pluggabiliy he imagined. We've learned that all driver implementations end up tightly coupling them.

As a result there is a bunch of complexity (and potential for bugs like #2921) that could be removed by simplifying the design.

This PR will attempt to do this.

• First step is already done: Fix WindowsDriver and CursesDriver to not use an Action for ProcessInput.
• Change all Action based events on ConsoleDriver/Application to use the proper EventHandler pattern
• Make MainLoop, IMainLoopDriver etc... internal implementation details, exposing curical methods/events on Application instead. Note this effectively means 3rd parties cannot build ConsoleDriver implementations; that's ok because nobody's ever done that anyway.
• Further simplify the console driver implementations by removing PrepareToRun and getting rid of more duplicated code.

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

[BDisp]

@tig please see my comments.

[BDisp]

@tig please see my comments. Thanks.