more seeking support. Still WIP.
add human-friendly(er) error description.
bugfix: due to a typo, IOSources where never seekable

even with explicitely asked to.
Add a property to export the seekabl-iness of an IOSource,
add basic unit tests to verify an IOSource is seekable when it should.
these tests are no longer expected to fail.

Now seek() should actually work.
very trivial (yet, correct!) strerror test.

Hey, it's a start; and it actually exercise the translator code
in a meaningful way.
error, testcase and fix when seeking in STREAM_ANY.