[3/4] msxml: Implement removeAttribute

[Mikołaj Zalewski]

  A problem with this patch is that the user may have an IXMLDOMNode * 
reference to the attribute node being removed. The xmlFreeProp will free 
the xmlnode so accessing the IXMLDOMNode could result in memory 
corruption. Under MSXML it's possible to access such node and even 
reconnect it e.g. to a different documents (at least I seem to remember 
such a thing for element nodes).
  I don't think supporting it is possible with our current reference 
counting only for the whole document. I was thinking about also keeping 
reference counts for each connected component and each node. The 
connected component count could be used to determine if the node can be 
freed in such a situation and the node count could be used to update the 
connected component counts if the tree gets split. But so far I haven't 
written any code (Another possibility is to keep in each node the 
refcount of the whole subtree starting in this node. This is more 
elegant but it would take a linear time with respect to the tree height 
to update the refcount).

Mikołaj Zalewski