more sensible SdrObject::Clone() and SdrObject::operator=()

Virtual operator=() is IMO pointless, and especially in a class hierarchy
like SdrObject it's pretty unlikely one could reasonably assign any
SdrObject-based object to any other one. Moreover, it was actually
only used in Clone(), which was almost never reimplemented, so the
more sensible choice is to have non-virtual operator= and virtual
Clone() always being reimplemented and using that.

This commit also fixes various smaller or bigger, er, interesting
details in the various operator= implementations.

sw/inc/dcontact.hxx

@@ -272,8 +272,8 @@ class SwDrawVirtObj : public SdrVirtObj
         // access to offset
         virtual const Point GetOffset() const;
 
-        virtual SdrObject* Clone() const;
-        virtual void operator=( const SdrObject& rObj );
+        virtual SwDrawVirtObj* Clone() const;
+        SwDrawVirtObj& operator= (const SwDrawVirtObj& rObj);
 
         // connection to writer layout
         const SwAnchoredObject* GetAnchoredObj() const;

sw/source/core/draw/dcontact.cxx

@@ -2272,20 +2272,21 @@ SwDrawVirtObj::SwDrawVirtObj( SdrObject&        _rNewObj,
 SwDrawVirtObj::~SwDrawVirtObj()
 {}
 
-void SwDrawVirtObj::operator=( const SdrObject& rObj )
+SwDrawVirtObj& SwDrawVirtObj::operator=( const SwDrawVirtObj& rObj )
 {
     SdrVirtObj::operator=(rObj);
     // Note: Members <maAnchoredDrawObj> and <mrDrawContact>
     //       haven't to be considered.
+    return *this;
 }
 
-SdrObject* SwDrawVirtObj::Clone() const
+SwDrawVirtObj* SwDrawVirtObj::Clone() const
 {
     SwDrawVirtObj* pObj = new SwDrawVirtObj( rRefObj, mrDrawContact );
 
     if ( pObj )
     {
-        pObj->operator=(static_cast<const SdrObject&>(*this));
+        pObj->operator=( *this );
         // Note: Member <maAnchoredDrawObj> hasn't to be considered.
     }