The attributes are now displyaed as columns of the tree
The first columns is the tag's name
The second is the value between "<tag>" and "</tag>" if the node doesn't
have any children, otherwise this column is empty
The others columns are for the attributes
Fix some bug related the height of the XML file and the maximum number
of attributes for a node in the XML file when creating the tree.
Fix bug when display the attributes of the selected node.
The tree is now updated if any attributes is added.
Modification of the way of getting the attributes to display when an
attributes is selected (now using the model instead of the xml element)