Moved the DTCPCleartextByteSeekRequest/Response classes into their own files & renamed them

Both classes were in rygel-dtcp-cleartext-byte-seek.vala which is
 against Rygel convention (which is 1 class per source file).

Renamed the request/response classes names consistent with the
 file names.

Added encrypted_length to the DTCPCleartextResponse constructors
 (for use by the DTCP/VOBU encrypted size estimation logic)
Merge pull request #10 from cablelabs/cbl_cpratt/cleartext-class-separation

Moved the DTCPCleartextByteSeekRequest/Response classes into their own files & renamed them