QString: don't detach in remove()/removeIf()/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:

• chop()
• clear()
• erase() (all overloads, eventually, cf. QTBUG-106182)
• fill()
• pop_back()/pop_front()/removeFirst()/removeLast()
• remove() (all overloads)/removeIf()
• resize()
• setNum() (all overloads)
• truncate()
• if QTBUG-106165 is implemented meanwhile: assign()