Allow the use of custom OnTouchListener

[fkirc]

In https://github.com/davemorrissey/subsampling-scale-image-view it is possible to assign a custom GestureDetector that works parallel to the gesture functions of the library.

Therefore it is possible to use code like this:

mImageView.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event){
                // detect swipe gestures or any other gesture
            }
        })

However, this is not possible with PhotoView, since it assigns its own OnTouchListener directly in the PhotoViewAttacher constructor.

I know that there are some helper methods like mAttacher.setOnSingleFlingListener(); or mAttacher.setOnDoubleTapListener();, but if would be much more convenient if the standard interface could be used.

Is there any plan to solve this issue?