STYLE: Refactor top-level CMakeLists into sections and improve reported messages

Report message are like the following:

```
-- VTKWikiExamples: Building VTKWikiExamples stand-alone
-- VTKWikiExamples: Building VTKWikiExamples stand-alone - yes
-- VTKWikiExamples: Setting executables prefix
-- VTKWikiExamples: Setting executables prefix - no
-- VTKWikiExamples: VTK VERSION: 8.1.0(OpenGL2)
[...]
```

or

```
[...]
-- VTKWikiExamples: Building VTKWikiExamples stand-alone
-- VTKWikiExamples: Building VTKWikiExamples stand-alone - no
-- VTKWikiExamples: Setting executables prefix
-- VTKWikiExamples: Setting executables prefix - yes [prefix: wiki]
[...]
```