lspclient: add rootfile pattern to detect rootpath
This makes it possible to use file patterns to detect the rootpath.
Though I should mention that the pattern will stay unused if the document was opened using open folder, as this already sets the rootpath, see here
For some reason, I was really feeling functional programming when I wrote this. If you don't like it, I can redo it in an imperative style (this would also enable me to pass that one QDir by const reference instead of value)
Edited by Eric Armbruster