Add DropArea item and Drag attached property.

Add DropArea item and Drag attached property.

Refactors drag API to improve compatibility with traditional drag and
drop by reusing events and adding drop actions.

Event sending is removed from MouseArea, instead the Drag object can
be attached to the item that is dragged and it will send drag events
when the position of that item is changed or when its active
property changes.

The DragTarget item is renamed to DropArea and can now communicate
supported and suggested actions.

Task-number: QTBUG-19747
Change-Id: I46cb77e68cf1ff32bbcbf0945facb593c9c2243c
Reviewed-on: http://codereview.qt-project.org/4638
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>