Python: Refine Python Inference APIs

[dmatveev]

Some parts of the G-API's Python API are not really python-ish, for example the infer interface:

..what could look like:

out = cv.gapi.infer(tag, g_in).get(0)
out1, out2 = cv.gapi.infer(tag, {'data', g_in}).get({'out1', 'out2'})

...or in dictionary form.