QString: don't detach in toUpper/Lower/...()

After we've detected that we need to change the container in toUppper(), don't detach(). Instead, check if we're shared. If we're not shared, do the change in-place, until we find that we need to grow the string ("slow case" in detachAndConvertCase()). Otherwise, or when we find we need to grow, make a new container, copy the subset that forms the final result over, then assign the new container to *this.

See epic for more details.

Acceptance criteria: The following member functions no longer detach:

• toUpper()/toLower()/toCaseFolded() (all overloads)