Make demo.cs a simple, pretty, demo - move 'tests' to UI Catalog or UnitTests

[tig]

Now that we have UI Catalog that allows quickly adding test scenarios that don't step on other tests, please move any tests out of the Example project (demo.cs) and add them to UI Catalog.

We should clean up demo.cs so it actually looks like a little app we'd be proud of. Right now it is full of cruft that either doesn't work, or is just haphazardly placed.

I've already moved Text Alignment and Mouse and will remove them from demo.cs as soon as the UI Catalog PR (#387) is accepted.

[BDisp]

I think we should maintain this project to have an alternative execution test. Because of this project I found a bug (#1613) that is not reproducible on the UICatalog.

[tig]

In a recent Reddit post on Terminal.Gui, a commenter complained that the "Terminal.Gui samples are horrible".

I asked him to explain what he meant and he said he looked at Example and that was the basis of his assertions.

I am going to submit a PR to address this issue that will

• Delete ./Example/demo.cs
• Add ./Example/Example.cs - using the contents of the simple c# example that is in ./README.md
• Update ./Example/README.md to note that the sample is intended to be a very simple example of using Terminal.Gui in C#. And to note that demo.cs has been deleted, but if people really still want it, it is in git history.

@BDisp, if you believe we should have an "alternative execution test" based on demo.cs please submit a PR that adds it, but ensure it's named in such a way that makes it obvious it is NOT an example, but a test app. Thanks.