[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