shared code into utility methods in nsContentUtils. Use the new methods to fix up nsContentAreaDragAndDrop to make it possible to drag image <objects> and image inputs. Bug 251775 and bug 244859, r=biesi, sr=jst git-svn-id: svn://10.0.0.236/trunk@160120 18797224-902f-48f8-a5cc-f745e15eee43