thumbor: Update to post v6.0

thumbor: Update to post v6.0

Catch package.accept_keywords up with dependency changes from the
current Gentoo tree and my recently-bumped dev-python/thumbor package.

Install the wtk overlay with some Layman hoop-jumping, since my
overlay has recently been bumped from the official list on account of
old EAPIs and such [1].  I'll work on getting it back up to speed, but
that's not going to happen today ;).

Also fix the location in the README tree (it should be under
gentoo-layman).

[1]: https://bugs.gentoo.org/show_bug.cgi?id=557812