QContiguousCache: missing move constructor

This container is missing the move constructor, and I have an idea why: the default constructor allocates, so the moved-from state would be different from the default-constructed (= empty) state, which, for a container, at least, isn't really acceptable.

Acceptance Criteria:

• Find out whether we can introduce a nothrow default (empty) state before Qt 7
  • If so, implement it, and add the move ctor to establish that state
  • If not before Qt 7, add a note or pre-program for Qt 7, so we don't forget
  • If not at all, implement with a partially-formed moved-from state (some functions already check for d == nullptr, e.g. the dtor, but it doesn't seem to be reachable from normal code so far).