Make 'comment' its own metaitem type

Change-Id: Ie663ab4c841e55159e36b3a3a9ca3381fb8037cf
ve.dm.Converter: Refer to nodes instead of elements where appropiate

Data elements are still elements (even in case of text nodes or
comment nodes), but DOM nodes are only elements if they're
HTMLElements. This should make it harder to accidentally call
getAttribute or some other Element specfic property that isn't
provided by the Node class.

Also fixed incorrect indexing of local addWhitespace method. Was indexed
as public method of this class.

Change-Id: Ie224aff469ccaec472eadd2ae6f02811d54d649a
Merge "Make 'comment' its own metaitem type"
Merge "ve.dm.Converter: Refer to nodes instead of elements where appropiate"