Provide support for file selection in preferences

[martin-fleck-at]

Feature Description:

The goal of this feature is to provide the ability to render 'string' preferences with a file/folder selection if they refer to a path. For instance, something like this:

VS Code does not provide something similar but I believe this is where the power of Theia customizations can come into play.
I can push a PR for this soon and would be happy to see if anybody is interested in this as well.