- added the dragable::on_drag_stop() event
- added the dragable::is_being_dragged() function
- Made the scroll_bar have a user settable style and made a default
  style
- Added some missing mutex locks to the scroll_bar widget

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402587