This method sets up the string gadget's stringinfo and stringextend
structures.  It allocates a buffer if needed and will use shared data
buffers for undobuffer and workbuffer if the maxchars is less than
SG_DEFAULTMAXCHARS (128).  Default text pens are: Foreground = 1,
Background = 0.  See the rootclass description of the om_new method for
more details.