Fixes #3974. TabView steals keypresses from active ContextMenu

[BDisp]

Fixes

• Fixes TabView steals keypresses from active ContextMenu #3974

Proposed Changes/Todos

• Add TabView KeyBindings for CursorUp and Cursor Down

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]

@tig is it possible to please prioritise getting this reviewed, it is the last blocker bug on getting next TGD edition out.

❤️

See gui-cs/TerminalGuiDesigner#307