Guard QDir::isRelativePath() from paths starting with ':'

Here we use isAbsoluteLocalPath() (in reverse, i.e. isRelative == !absolute); This is sort of an addendum to 30abfc8c.

Note that the upstream QDir code implements isAbsolutePath() in terms of "it's absolute if it's not relative". It has the same issue with paths starting with ':', since ultimately it uses the QFileSystemEngine backend which treats paths starting with ':' as absolute paths since it considers such paths as denoting a Qt Resource (qrc).