This allows to distinguish Processor::configure_io from the
Plugin API with the same name. Despite the identical name,
both functions serve different purposes.
Likewise the ::can_support_io_configuration() API is renamed.
The signature of that function call has already diverged.
The Plugin-API has to handle optional busses (e.g. side-chain)
and replication. The PluginInsert processor provides context.
This change is in preparation for AudioUnit and VST3 busses.
Currently a stereo-input (1 bus) cannot be distinguished from
mono + sidechain (2 busses).
10b05df189