Adds ViewportSettings.Transparent

[tig]

Fixes

For

• Popover - A consistent way of enabling a Subview to popup outside of a View #3691

We need the concept of a view that is transparent. In addition I've found that View.Draw has some errors in it regarding in how clipping is done.

This PR fixes those issues and adds the new capability.

• Adds ViewportSettings.Transparent enabling see-through/click-through views
• Adds ViewportSettingsEditor to Scenarios and expands AllViewsTester to utilize

Proposed Changes/Todos

• Add ViewportSettings.Transparent
• Add ViewportSettingsEditor to Scenarios and expand AllViewsTester to utilize
• Upgrade ViewportSettings scenario to use ViewportSettingsEditor and illustrate transparency
• Add ViewportSettings.Transparent 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]

Note, it's unlikely that this will support a transparent view with subviews that use transparent shadows.

[tig]