editor: miscellaneous source cleanups.

editor: miscellaneous source cleanups.

Apparently I didn't know about chained comparisons when I wrote this
code, so I didn't take the opportunity to write '0 <= x < limit'.

Also, removed the wildcard import from tkinter, replacing it with
explicit references to the things needed. There weren't that many of
them, so I think the 'import *' didn't gain much.