MouseArea shouldn't grab the mouse until there is an effective drag.

A MouseArea shouldn't prevent a parent MouseArea or Flickable from
handling a drag event unless it is going to do something useful with
it.

(cherry picked from qtquick1/67db779665b1d95a20720c0dee058c47f7df8726)

Task-number: QTBUG-29717
Change-Id: I17caa51ebc2e547e73e727fd185e60644591ad2b
Reviewed-by: Alan Alpert <aalpert@rim.com>