Opera (<=12) lacks HTML5 Drag and Drop support

[wilkinson]

On my test machine, which is running Mac OS X 10.7 "Lion", the latest stable and even the alpha versions of Opera still lack support for drag and drop. Consequently, dragging an image onto the page redirects the browser to the local file instead. Although the majority of users will have access to other browsers, ImageJS should still warn users of the "impending failure". Ideally, ImageJS would degrade gracefully with some alternate input method in Opera.