Support scroll bars for messages edition

Currently, there are unexpected behaviors when moving a message while the diagram is scrolled down.

Edit policies get the mouse location relative to the corner of the diagram, where the logical model works with the absolute positions. A simple fix should be applied to the location of the requests to get the right position in terms of absolute position and not screen relative.
Another similar issue should also be tracked for scaling issues, when the diagram is not zoomed 100%.