Changed how we generate and load protobuffers

[gustavo-iniguez-goya]

On 5dd4ef0 and 6f41dc6 the way we generate and load the protobuffers has changed, to support multiple python3-grpcio / python3-protobuf versions.

Now the protobuffers default location for the GUI is opensnitch/proto/ instead of opensnitch/

For now, the compiled protobuffers has been added to the repo, but they can be moved outside of the GUI directory.

@lsfxz , the PKGBUILD file needs to be updated:

in build(), instead of sed -i ... opensnitch/ui_pb2* now it should be find opensnitch/proto/ -name 'ui_pb2_grpc.py -exec sed -i ... {} ;`