Support C++11 move semantics where applicable

DL;DR: all copyable Qt classes should be (nothrow-)movable

To this day (Qt 6.4), there are still Qt classes which have a user-defined copy Special Member Functions (SMFs), preventing implicit generation of move SMFs. These types are therefore not movable, in particular, they're not nothrow_move_

, 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) to all copyable classes.

Acceptance criteria: For each copyable class C:

• either C
  • is nothrow move-constructible
  • is nothrow move-assignable
  • has a nothrow member-swap
  • has a nothrow ADL (free function) swap
    • Q_DECLARE_SHARED provides that
• or
  • has a code comment explaining why it doesn't