Removed "New tabwidth" menu binding.

Added "a tab means how many spaces?" dialog to block tabify and untabify.  I
think prompting for this is good now:  they're usually at-most-once-per-file
commands, and IDLE can't let them change tabwidth from the Tk default
anymore, so IDLE can no longer presume to have any idea what a tab means.

Irony:  for the purpose of keeping comments aligned via tabs, Tk's
non-default approach is much nicer than the Emacs/Notepad/Codewright/vi/etc
approach.