Widgets: drop onto closed sidebars.
- Make dropping a widget easier by preventing the source sidebar from resizing while the widget is being dragged.
- Move widgets dropped on closed sidebars to the top. UI Sortable places them randomly near the bottom.
- Fix possible regression in wp_list_widget_controls(), add an optional argument to output the sidebar name inside the sortable container.
- Updated styles for sidebar description and widget-hover class.
Part props shaunandrews, fixes #25952.

git-svn-id: https://develop.svn.wordpress.org/trunk@26426 602fd350-edb4-49c9-b593-d223f7449a82