Fixes #3318. v1-MouseClick should be invoked only once on a mouse click.

[BDisp]

Fixes

• Fixes v1-MouseClick should be invoked only once on a mouse click. #3318

Proposed Changes/Todos

• Only consider clicked event of any button
• Added unit test for proof

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

[dodexahedron]

Thread safety and performance comment for the new code and a terminology nitpick for the test method.