[resources] Use "/" instead of QDir::separator
Summary:
As Qt documentation states (https://doc.qt.io/qt-5/qdir.html#separator),
QDir::separator() should not be used to build file paths.
Note to reviewers: I might have confused QStringLiteral vs QLatin1String vs QLatin1Char
in some places, please correct me if I'm wrong
Test Plan: It compiles
Reviewers: dvratil, mlaurent
Reviewed By: mlaurent
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D28634