Add specialization for Set<QString>::toString(value)
Add specialization for Set<QString>::toString(value)

This allows us to call Set<QString>::toStringList(), e.g. for debugging
purposes.

Change-Id: I6bf24e6e618bc95d677bc442a8032b586e27f43a
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@qt.io>

qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs