TextFieldTreeCell builds a TextField over the graphic part of the TreeCell

A TreeCell defines a graphic and a label.
The Labeled part can be editable using the TextFieldTreeCell.
But when you double click to start editing, the TextField is located over the graphic.

I think this is not correct, and the TextField should be displayed on the editable part only, that is the Labeled.
This will avoid to get the editable text shifted to the left when we start editing, then shift back to the right when we commit.
See attached screen shot + simple test case attached.