Eliminate -Wcast-qual warnings with GCC and Clang

Use const_cast, mutable, and various other changes to avoid -Wcast-qual

samples/dnd/dnd.cpp

@@ -2022,7 +2022,7 @@ void DnDShapeDataObject::CreateBitmap() const
     m_shape->Draw(dc);
     dc.SelectObject(wxNullBitmap);
 
-    DnDShapeDataObject *self = (DnDShapeDataObject *)this; // const_cast
+    DnDShapeDataObject* self = const_cast<DnDShapeDataObject*>(this);
     self->m_dobjBitmap.SetBitmap(bitmap);
     self->m_hasBitmap = true;
 }