Go back a slide on pressAndHold

While a gesture would be a lot better, pressAndHold is an easy to implement
way of providing limited 'go back' functionality on a touch only device.

Change-Id: Ic6b2f4759dc5c7bdcb84d9dc5ef7c40589746c8d
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>

src/Presentation.qml

@@ -170,6 +170,7 @@ Item {
             else
                 goToNextSlide()
         }
+        onPressAndHold: goToPreviousSlide(); //A back mechanism for touch only devices
     }
 
     Window {