Widget Factory _on() could make it easier to apply multiple handlers

Widget Factory's _on() method, because it uses a map to accept handlers, makes it clunky to apply multiple handlers to the same event-selector combination. You have to do:

this._on($element, { 'click .class' : functionOne } );
this._on($element, { 'click .class' : functionTwo } );

If the map values could be arrays of functions, that would be very convenient:

this._on($element, { 'click .class' : [ functionOne, functionTwo ] } );