Replacement for QWS_DBLCLICK_DISTANCE

Add QT_DBL_CLICK_DIST to replace QWS_DBLCLICK_DISTANCE for controlling
the distance for detecting double clicks, which can be very useful on
embedded devices

[ChangeLog][QtGui] environment variable QT_DBL_CLICK_DIST 
customizes the amount of movement allowed when detecting a double click.

Task-number: QTBUG-40841
Change-Id: I0a7534ad6cd6387d127eb49021a92d414d45670e
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>