QUrl implicit sharing is not thread-safe

There are several const methods in QUrl which mutate the QUrlPrivate structure via calls QUrlPrivate::parse() without detaching first. For example, QUrl::toString(). This means that there may be data races when these methods are called from separate threads on two different QUrl objects where:

1. Both QUrl objects share the same QUrlPrivate structure implicitly and
2. QUrlPrivate::parse() has not yet been called.