Linux: Add drag&drop support (issue #1258).
- Requires proper handling of the "XdndProxy" property on the top-level window. This currently works in cefsimple but not cefclient (perhaps due to https://bugzilla.gnome.org/show_bug.cgi?id=653264).

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk/cef3@1759 5089003a-bbd8-11dd-ad1f-f1f9622dbc98