Align SID frame size

After deciding on a common SID frame size of 5.2kbps, it would be good to have it at least mechanically harmonized soon, since there is work on DTX for the ISM modes ongoing/planned which would be much simplified when having the final SID size already in the codec. We propose to first implement a simple solution that

• adds the SID frame header in all modes (currently, I think the SBA 5kbps SID does not have it)
• only uses one SID frame size of 5.2kbps by adding padding bits in the SID

So, this version would not change functionality or break bitexactness, but only change the size of the SIDs. I can start a branch and begin the work at least for stereo, but may need input from the involved parties for their respective operating points later.