Fixes #3090. Introduce StatusItem.HotTextSpecifier

[AndreyAkinshin]

Fixes #3090 - Include a terse summary of the change or which issue is fixed.

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]

Nicely done.

So we're clear, I'm breaking the rule of "no new v1 APIs" by supporting this. I'm doing so for several reasons: I love drawing in new contributors who are willing to do the work to submit PRs. And StatusBar is super-legacy and will die a slow death in the v2 world, so who really cares ;-).

But this does need new unit tests before I'll approve it.

[AndreyAkinshin]

So we're clear, I'm breaking the rule of "no new v1 APIs" by supporting this.

Thank you, much appreciate it!

But this does need new unit tests before I'll approve it.

A unit test is added.