function _getPaneFromElement($el) {
        return _.find(_panes, function (pane) {
            if (pane.$el[0] === $el[0]) {
                return pane;
            }
        });
    }
Retrieves the Pane ID for the specified container
@param {!jQuery} $el - the element of the pane to fetch
@return {?string} the id of the pane that matches the container or undefined if a pane doesn't exist for that container