Add 'project-relative' as value for 'xref-file-name-display'

Add 'project-relative' as value for 'xref-file-name-display'

* lisp/progmodes/xref.el (xref-file-name-display): Document new value.
(xref-location-group ((l xref-file-location))): Handle the new value.
(xref--project-root): Extract from the default method of
'xref-backend-references' so it can be used in above's new code.
Also fix an old bug in the "backward compat" branch.

* lisp/progmodes/xref.el (xref--project-root-memo): New variable.

* test/lisp/progmodes/xref-tests.el: Add test cases for the three
possible settings of 'xref-file-name-display'.

Co-authored-by: Tobias Rittweiler <trittweiler@gmail.com>