Make the screenshot drag-and-drop support require a modern HTML5 browser..

The screenshot drag-and-drop support required Google Gears before, but the
remote file upload and drag-and-drop support was broken since Firefox 3.5
came out, and never fixed. We now use the new APIs in modern HTML5 browsers.
This works in Firefox 3.6 and should work in newer Google Chrome/Safari
browsers, but haven't been tested.

Tested uploading with Firefox 3.6.