[TIMOB-1624] iOS: TiDOMNode nextSibling, previousSibling properties skip non-element nodes

The nextSibling and previousSibling properties on a DOM node are supposed to return whatever the next node is, and iterating along them should always give the same results as iterating over the parent node's children property nodelist.

However, they have been implemented on iOS by calling libxml2's xmlNextElementSibling and xmlPreviousElementSibling functions, which skip over non-element nodes.

This can lead to interesting failures trying to iterate through a mixed element/text/cdata set, and causes big red error screens on the 'XML DOM' demo in KitchenSink when the 'nodewalker' test functions do exactly that.