What is the difference, conceptually, between TextMorph's and 
StringMorph's?  And, while we're at it, PluggableTextMorph's?

A tentative answer:
TextMorph	intend to hold formatted text and other morphs.  This has 
aspirations to be a word processor.

StringMorph	plain old text (except even that can have fonts, emphasis, etc, 
can't it?)

PluggableTextMorph  most salient feature is that it is a ScrollPane, so you 
get scrollbars.

Putting the question more practically, in what circumstances is it 
appropriate to use each?