Removed Polish translation

Last updated in 2002 (that's 12 years ago) and with 68 strings translated,
it's doing more harm than good at this point. It can always be brought back
from git, but it's probably easier to just start from scratch, if anyone's
interested.
Added dummy window as parent in warning and critical error handlers.

In gtk 3.14, creating a message dialog without a parent window
causes a crash.  Unfortunately this causes the original error or
warning message from being seen by the user.
Removed obsolete tab-hborder property from media-select UI file.

Unfortunately, beginning with gtk 3.14, rather than ignoring this obsolete
property, having it in a UI file is considered a critical error, thus crashing
glabels on newer distributions.