This fixes a bug that can result in inconsistent session-state when
copying plugin state from one plugin to another (via drag/drop
ProcessorBox::object_drop, LINK).
The underlying plugin state and settings are copied, port _shadow_data
is updated, and ::get_parameter() shows the correct new value.
However the Controllable was not updated. On Session save/restore
the value may have be lost or was inconsistently restored.
430e51065c