Use multiprocessing instead of multithreading

[pozitronik]

Because of the GIL the GUI model does utilize only one CPU core. The processing threads and display thread virtually shares the same process, which leads to visible lags. It doesn't affect that much in the case when ONNX framework is used (it seems it has own multiprocessing implementation), but main processing thread still affects program latency.
Solution: rewrite GUI Model to true multiprocessing support.