Enable comparing KFileItems by url

Summary:
Based on QUrl comparisons (available since Qt 5.4).
Those comparisons can be used to speedup the lookup of the requested KFileItem, for example using std::lower_bound.

Test Plan: Included a new test in kfileitemtest

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: bruns, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13211