Added move of comments to nodetype.module

I made it for my own needs, but maybe it will interesting for someone else.
I moved default Change Node Type form field into fieldset, as it was mentioned in other issues.
Also I added that similar simple functionality for comments.
When you edit the comment, You have option to move it to other node by entering this node {nid}
Only node {nid} is changed.
However, after that, the _comment_update_node_statistics() is called for both affected nodes to keep it updated.
Also, there is simple validation for unexistent node {nid}