Fixed: Bug 62794 - [L3] no drag and drop of pictures while composing a new mail

Root cause: Dropping images to an iframe caused the browser to reload the whole view which might lead to dataloss. Since no easy fix was found, we disabled d&d
Solution: Reenable d&d by attaching listeners inside the iframe which will prevent reload of the page with the dropped content on the one hand and on the other hand correctly uploads image based on the previous mechanisms