As of release based on tk4.1 this 'mTk' directory 
contains _complete_ sources of Tk and Tix in their minimally modified
forms.

This is because 'munging' to pTk directory from either modified form
or original Tk becomes very complex now Tk is split unix/generic.

It should also makes merging changes less error prone.

The mac, win and xlib directories may need attention too.
