Simplify the AudioPath name generation

Remove the duplicated "indexed" path name generation and just let the
indexed path types generate the name themself. This also make things
simpler for future types that might need another naming.
Add dedicated outputs for the left, right and center buses

The bus outputs make it possible to use the samplers in setups using
an external mixers.
Merge pull request #66 from AlbanBedel/output_buses

Output buses [was bug #970965: Allow sending the crossfader inputs to an external mixer]