* Introduce BPackageWriterParameters which comprises all parameters
for package creation, currently flags and compression level. Such an
object can be passed to BPackageWriter::Init() and is passed on to
PackageWriterImpl and WriterImplBase.
* PackageFileHeapWriter: Add compressionLevel property and pass the
value on to ZlibCompressor.
* package add/create: Add options -0 ... -9 to specify the compression
level to be used.
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