options for dragAndDrop to control it with left-mouse-button and one-touch

Hi Johannes,

thank you first for the great job you did with ocanvas. Now I'm using it in one of my projects and I needed to patch a little the dragAndDrop-method in order to use the other buttons and gestures for other functions like zooming, etc. It would be nice, if you integrate this changes in the next version or just consider such needs in your concept.

Kind Regards,
Iva