While I was at it, I noticed that the ASCII chart and hex dump
viewer windows were always in front of the main app window. It looks
like child windows are always in front. The easy fix is to not
set an owner.
This causes a new problem: the windows don't get closed automatically
when the parent window closes, and the app won't exit until all
windows are closed. So we now explicitly close the hex dump and
ASCII chart windows when the main window is closed, and we now keep
track of all the external-file hex dump windows. (It always sort of
bothered me that we were creating hex dump windows and not keeping
track of them. Itch has now been scratched.)
Also, changed the ascch-mode setting to be an enum rather than an
integer. Renamed the setting to ascch-mode1.
