Fixes #2954. Modal view is always refreshing only by moving the mouse.

[BDisp]

Fixes #2954 - The current and unique modal toplevel must clear all the terminal size and not only itself if it's dirty.

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]

LGTM except unit tests are failing.

[BDisp]

LGTM except unit tests are failing.

It's the TestTreeViewColor again. If I run it alone it pass. I'll see what going on.

[BDisp]

LGTM except unit tests are failing.

It's the TestTreeViewColor again. If I run it alone it pass. I'll see what going on.

Fixed, I had to force shutdown on my unit test.