Add panBoundsPoint property to TransitionGestureHandler

[felipowsky]

I was looking for a way to configure an end point to the pan gesture so the interactive transition would look smoother.

Imagine that you have a targetView that its position starts at the bottom of the screen with a pan gesture handler configured with top direction and ends at the center of the screen when transitioning to the next viewController. In current implementation the user would need to pan until the top of the screen and the targetView would look misplaced compared to the user's finger.

This PR fixes that by introducing a newCGPoint property to TransitionGestureHandler called panBoundsPoint.
When panBoundsPoint is set the percentComplete will take into account the panBoundsPoint as the endPoint of the transition instead of the targetVC.view.bounds.

To be honest I'm happy with the results but it seems that there's a best way to handle this.
Anyway it was the solution I came up with now.

Let me know what you think.

[xxxAIRINxxx]

@felipowsky
Great!
Thanks 👍