convert in style-list%

Converts an external style to a style in this list.

• (send a-style-list convert style) -> style<%> object
     style : style<%> object

  Converts style, which can be from another style list, to a style in this list. If style is already in this list, then style is returned. If style is named and a style by that name is already in this list, then the existing named style is returned. Otherwise, the style is converted by converting its base style (and shift style if style is a join style) and then creating a new style in this list.