Drag n drop (#223)

Added drop interface for image files, images, and text, dragged from other applications.  (#153,#223)

- Lock aspect ration on new image objects by default
- Communicate context menu click location to paste actions

model/ModelImageObject.cpp

@@ -73,6 +73,8 @@ namespace glabels
 			{
 				smDefaultImage = new QImage( ":images/checkerboard.png" );
 			}
+
+			mLockAspectRatio = true;
 		}