addrelease: Try harder to seed the RE properly

Disc numbers <= 0 were previously changed to values lower than 0 which
caused the RE to crash. Fix this by using the lowest disc number of the
cluster (which is somewhat sorted) to figure out the difference between
that disc number and 0 and use that number to change all disc number to
be 0-based while preserving their order.
Hide unsupported files in the file browser

This fixes PICARD-395 because unsupported files are not shown anymore.