QSet: don't detach in remove()/erase()

After we've detected that we need to change the container in remove()/erase(), don't detach(). Instead, check if we're shared. If we're not shared, do the change in-place. If we are shared, make a new container, copy only 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:

• clear()
• erase() (all overloads, cf. QTBUG-106180)
• intersect()
• remove()
• subtract()
• unite()