design and implement audio ports configuration extension

[atsushieno]

Current AAP design was partly inspired by LV2, and ports are such ones. After some experiences, it turned out that audio ports should work along with audio bus adjustment. For various effectors ports wouldn't matter i.e. for mono outputs one float channel is processed, and for stereo outputs two float channels are processed on the same function. It is up to the plugin.

LV2 ports are inflexible, but AAP does not have to follow the same way. It is still up to host how audio buses are implemented, but at least current AAP ports structure needs redesign:

• remove audio I/O ports from aap_metadata.xml
• add audio bus negotiation in the API ("do you support stereo?" "no? how about mono?" etc.)
• negotiate CV ports as well?

[atsushieno]

Since Redesigning ports involves the entire internal AAP protocol changes in AIDL, we should consider this as a next major upgrade (even "AAP2").