Added stopPropagation to main event

[haas85]

There is a call in the main event that calls to srcElement's prevent default. This commit do the same but for stopPropagation.

data.stopPropagation = function() {
data.srcEvent.stopPropagation();
};

[runspired]

This is likely to support something along the lines of #808. Overall, this may be a bad approach currently because srcEvent isn't always the event you think it is (e.g. in the case of pan, srcEvent is currently a mousemove not a mousedown). Should be wrapped into changes to fix #815.