Added support for SDL_DROPFILE event. Input system sends event with filename only, and UI subsystem interprets that to include the element that was dropped onto. The UI variant of the event is only posted when the OS mouse cursor is visible (such as in the editor.)

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2683 3bd170d8-ddcf-f82c-f583-7a401ccf5608