Drag & Drop "move to folder"

* Drag & Drop "move to folder" preliminary implementation
* Use "drag handle" button to initiate drag
* Use the message list item itself as the drag thumbnail for
  one item; "Move N messages" for multiple (not implemented)
* Disable MailboxList update during drag
* Use ListView itself as drag/drop target

Bug: 3138004
Change-Id: I9243e4f583cf91caa1b9dd04fd64d52e8b28fa30
6 files changed