Support colspan -1 & 0
When #7490 will be implemented, there will be no way to "remove" a tag from the form view. Sometimes it may be needed but as #7490 explain, removing is not a good way. Indeed I think the best way is to keep the tag in the view but does not render it at all. This is different from the 'invisible' because this one takes spaces.
I propose that a negative colspan means that the widget is not attached to the table (and even no created). This is quiet similar to 'tree_invisible'.