kax_analyzer: remove empty seek heads

Trying to UpdateSize()/write an empty seek head results in an assertion
from libEBML (it also doesn't make sense). So remove instead of trying
to write them.

Fixes #1007.
debian/control: change section to »video«

See #1004.
debian/docs: include full ChangeLog in package

Implements #1004.
Translations: update Polish
Build system: adjust po settings for Polish translator's poedit
debian/rules: only build mmg, mkvinfo for the GUI package

Those two applications are the only ones that are kept before a »drake
clean« directly afterwards and a full re-build. So don't waste the time
building anything more than necessary on the first run.
Translations: add Brazilian Portuguese
Translations: integrate Brazilian Portuguese