Display relative path (including name of the checkout) in the titlebar

Summary:
My wife requested this a very long time ago ;)
Thanks to Christian Ehrlicher for the hint on how to implement it.

BUG: 391804

Test Plan:
While navigating in a checkout, the titlebar shows
  <name of checkout>/relative/path

Reviewers: chehrlic

Reviewed By: chehrlic

Differential Revision: https://phabricator.kde.org/D11355