Drag Feedback And MailNews Fixes

MailNews: Clicking a column header no longer switches threading off (Bug 397928) and opening a message in an existing window correctly refreshes the view now (Bug 173228)

General: Drag feedback for text and images (like browser tabs or messages from the MailNews list) has been available for Mac and UNIX/Linux since March (Bug 178513) and is now also available on Windows (Bug 374593). Previously, such items were represented by a simple small rectangle near the mouse cursor. On GTK platforms such as Linux, the drag feedback is still opaque, though (Bug 376238).