Fix margins for right-to-left mode

Fix margins for right-to-left mode

The behaviour for considering left and right margins was inconsistent
in views with a right to left layout; these values were reversed for
extent calculations but not for general positioning. With this change
the left and right margins are never reversed in a right-to-left layout,
so minXExtent and maxXExtent calculations always use startMargin
and endMargin respectively, regardless of layout direction.

Also fixes calculation of endOffset in trackedPositionChanged() when
in horizontal orientation.

Change-Id: Ie00e3d4c2bd38d8fe6ac0213702206b88bfa895e
Reviewed-by: Martin Jones <martin.jones@nokia.com>