QML Settings: Check for and warn about unsupported URL schemes

Currently, the Settings.location is checked by calling `QUrl::isLocalFile()`. If it's not a local file, the URL is ignored: https://github.com/qt/qtdeclarative/blob/v6.5.1/src/core/qqmlsettings.cpp#L239

Ideally, the user should receive some kind of warning instead of having their URL silently ignored (say, if they tried to put a read-only INI file in a QRC resource)