AURORA: Remove BIFFile::mergeKEY() / BZFFile::mergeKEY()

We need to restructure how KEY/BIF works for Phaethon.
AURORA: Rewrite KEYFile to be an Archive

For xoreos, we load BIFs as archives and merge the
information contained in the KEY files in. However, in
Phaethon, we want to examine the KEY files instead.
So we need to reverse this.
MAIN: Load KEY files as archives

No file size (or data reading) yet, since this information
is in the BIF/BZF files.
AURORA: Add a base class for KEY data files
AURORA: Rewrite BIFFile to be a KEYDataFile
AURORA: Rewrite BZFFile to be a KEYDataFile
AURORA: Extend KEY to receive KEYDataFiles

Now reading key resource sizes and data should work.
MAIN: Load KEY data files