The QDom classes have user-defined copy Special Member Functions (SMFs) and a user-defined destructor, which disables the implicit generation of move SMFs. These types are therefore not movable, in particular, they're not nothrow_move_{constructible,assignable}, so they perform more poorly than they could in many situations, such as returning from functions and reallocation of containers.

Add move SMFs, possibly member-swap and free swap functions (by using Q_DECLARE_SHARED, if that's BC these days).

Acceptance criteria:

• All QDom value-like classes (incl. QDomNode hierarchy) have
  • an inline noexcept move constructor
  • an inline noexcept move-assignment operator
    • just swaps, not move-and-swap
  • an inline noexcept member-swap, if possible
    • probably need a way to prevent mixed-type swapping
      • see QPixmap/QBitmap for a technique
    • likely move-assignment needs the same protection
  • are marked as Q_DECLARE_SHARED, if possible
    • adding Q_D_S after release used to be BiC in Qt 5, check the situation in Qt 6, add Q_DECLARE_SHARED_NOT_RELOCATABLE_UNTIL_QT7 if Q_D_S is still a problem