When you want to add a text child node, you must type some text in the
little box, because if you don't, no node is created. But this little box
isn't the best way to enter text, because we have a node editor to do that,
and this node editor will eventually be enhanced ; and if the user clicks
on "Validate" with no text in the box, nothing happens : that can be
surprising.

I think we should remove this little box in the node creation dialog when
TEXT NODE is selected, and we should simply create an empty text node which
should have focus, so the user would be directly able to type his text in
the node editor.

I think we should avoid all situations where the user does something to
perform an action and where nothing happens. As an example : if you click
on "Add child node" when the current node can't have any child, we should
warn the user, and say why he can't perform this action. This avoids
surprises and unreactivity of the interface.