KUrlNavigator: Decode url title fully

This commit fixes symbols like "'" in the url title parameter being displayed as "%27".

https://invent.kde.org/-/project/2775/uploads/9d77f1ff25b69ec480925e5e56d27f61/grafik.png

This has become a problem because we started to percent-encode more characters to support the display of some non-latin characters. However, KUrlNavigator until now didn't fully decode every such percent-encoded character. See system/dolphin@2059ce29 and system/dolphin!806 (merged).