Drag Attachments Changes in TB15

The new framework that 15.0.500 is using requires us to commit to either a full-native drag or an internal one as soon as the drag operation starts and can't be changed on the fly, so we added drag handles on Windows for dedicated reordering purposes. macOS already worked like this too before, but we had to change the Windows implementation to work the same way.

Drag the handles to reorder, or the attachment itself to start a native drag.