Fix dragging of selections

A regression from (r151091) broke drag and drop of selections. This
patch restores the maintenance of the m_writableData which is used
by DragClientQt.

Task-number: QTBUG-37314
Change-Id: I9afa193daec53d8d5affa560b8f225bb7dc79470
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>