Simplify autogen.sh and remove legacy files

ChangeLog was unchanged since 2006. We have version control.
TODO was fairly random. Somewhat valid, but clearly not up to date or
reflecting current priorities.
Further cleanups to directory structure etc

Still need to resolve the warnings from glib-gettextize.

By using (non-glib)gettextize and the accompanying AC-macros, this seems to
work but introduces a world of pain in annoying build artefacts. Some of it
is apparently possible to work around (like automated ChangeLog updates)
but this needs to be done properly.