StemSeparation: Write back end selector

[kcoul]

Dev:

• Detect and make available all possible back ends for given platform
• Allow back end switching from binary (runtime flag) or plug-in (GUI)
• Make available benchmarks to determine ordering from fastest to slowest back end

Prod:

• Build using fastest and/or best output quality option for each platform

NOTE: Options should be expanded to include GPU/CPU models (tflite, onnx) of other Stem Separation engines to make an exhaustive list