ensure clangd is started for each project on it's own

Summary:
If you work on different projects, at the moment clangd is shared.
That doesn't work, as it needs a compilation database "per project".
Therefore: move the heuristic "what is the root" to the generic code
and use the root for the compilation database

Test Plan:
now, I get proper handling if I open different projects at once
before, the first project did win and all others did have
wrong compilation databases used

Reviewers: dhaumann, mnauwelaerts

Reviewed By: mnauwelaerts

Subscribers: kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D22710