0028987: Visualization, SelectMgr_SelectableObject - move out iterator from object

0028987: Visualization, SelectMgr_SelectableObject - move out iterator from object

Added method SelectMgr_SelectableObject::Selections() and depreacted methods
Init(), More(), Next() and CurrentSelection().

Added method SelectMgr_Selection::Entities() and depreacted methods
Init(), More(), Next() and Sensitivity().

SelectMgr_SelectableObject::myAssemblyOwner has been moved to AIS_MultipleConnectedInteractive.
SelectMgr_SelectableObject::Selection() now returns NULL handle for not found selection.
SelectMgr_SelectableObject::HasSelection() is no more virtual and just retuns !Selection().IsNull().

SelectMgr_SelectionManager::mySelectors map is now declared using proper
key type Handle(SelectMgr_ViewerSelector) instead of Handle(Standard_Transient).