Fix parsing of sound and image urls with relative paths

Summary:
Relative file paths for sound and image files were not correctly parsed
into QUrl objects. The reason for this is, that the previous used
QUrl::isRelative() does not work as its name lets suppose. So the
relative file path detection is changed to QDir methods.

Unit tests for sound and image file path parsing were added.

BUG: 389517

Test Plan: Run unit tests

Reviewers: #kde_edu, whiting, tcanabrava

Reviewed By: #kde_edu, tcanabrava

Subscribers: kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D10370