Added missing destroy() to Meio.Element.List

Using '.destroy()' on a Meio.Autocomplete instance results in:

> TypeError: this.elements[element].destroy is not a function

in Meio.Widget.destroy(), because there is a destroy() function in Meio.Element.Field but not in Meio.Element.List. 

This fixes it, only drawback is, that the list element will be destroyed, even if it's shared with different instances.