Test C# theia plugin with multi-root workspace in Theia

[tsmaeder]

We need to make sure omnisharp works ok with multi-root workspace, using version 0.0.4 from https://github.com/redhat-developer/omnisharp-theia-plugin/releases

[tsmaeder]

Did some tests, in particular:

1. started a workspace with the C# and the suggested default project
2. Dependencies are restores, language features work fine
3. Switched to "multi-root" in the preferences
4. Added some random folder as a second workspace root
5. Refresh the browser to restart the IDE
6. language features work fine
7. Remove the default project as a workspace root
8. Refresh the browser
9. Add the project as a workspace root
10. Observe: language features do not work in the project
11. refresh the browser
12. Observe language features work in the project

[tsmaeder]

My conclusion is: the php plugin works with workspace roots, but it does not handle changing workspace roots. We would need that support in order to merge the fix for #13427.

[tsmaeder]

Also, omnisharp can only handle one open solution at a time. Microsofts C# extension offers support to chose an "active" project (https://code.visualstudio.com/docs/languages/csharp#_roslyn-and-omnisharp). We don't seem to have that.

[tsmaeder]

Opened issues to fix the problems uncovered.