Web Component mixin that makes elements pannable. By default it will add mouse
event listeners to the host element and apply a CSS transform to it whenever it's
dragged. You can change the element that the tranform is applied to by calling
setPanningElement(). Note that this doesn't change where the mouse event
listeners are applied.
You may reset the panning position (i.e. return the element to it's regular
position) by calling resetPanning().