clarify simplify folders function (partial)

WIP:  splitting the simplify function into parts.  Improving
cross-platform-ness.  Clearer structure in many places.   Expected
function similar, but more readable and robust.  NOTE - still need to
clear second layer of empty folders, bottom-up walk?
finish simplify folders upgrade

now removes empty folders properly, doesn't delete everything (!),
silently removes 'Thumbs.db' (expandable to other platform annoyances)
groundwork for overlapping changes (orange)
comment line merge section

because I was going to have trouble the third time, which means I need
more comments.
cleanup return values in merge function

A collection of lines if successful or empty string on failure can be
truth-tested directly

Also logging installed mods
fixed invalid chars bug, crash bug, logging

Invalid characters to merge - which I *think* might include accented
chars - are now simply dropped, so the merge  compatibility is low but
better than a merge crash!  Files added now check if they'll overlap the
mixed folder, not mod folder - merge rejected every time.  The last
action of a mod merge is now to write it's name in the installed mods
log.  Minor style cleanup.
also log version of DF

And a comment at the top.  If this ever gets read
merge_seq docstring fix
version 0.4 and documentation

Some goals achieved, others clarified.  General documentation
improvements.

Tweaked insert blank files func to work on object folders as if
replacing rather than adding that folder (as documented)