Introduce Q_DECLARE_SHARED_NOT_MOVABLE_UNTIL_QT6

... with classical semantics, meaning movable-come-Qt6,
and complex-until-then.

Whether or not we want a new flag for movable-except-
in-QList is an orthogonal question, and should not hold
back the slew of commits that introduce use of this
macro.

Change-Id: I3a6be08c314fcd7f3315af138625c38196debda5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>