Refactor the package

[PiRK]

At some point the ElectronCash package got refactored to make it possible to execute it from a simple script. This may seem like a good idea for development purposes, but it has the side effect of requiring some auto-generated files to be part of the source package: protobuf compiled file, icons.py, libsecp256k1 (.dll or .so)...

I would like to switch back to a package structure where only source files written by humans are part of the repository, and other files are generated only when installing or generating binaries.
It will still be possible to run the program from the development repository, using an intermediate script that builds the necessary files in a temporary build directory and patches the Python path to include the build (see https://github.com/silx-kit/silx/blob/master/bootstrap.py)

This will be a good opportunity to rename the subpackages, and merge them into a single package.

The current structure of the project is a loose bundle of 3 packages and many resource files:

ElectrumABC -- electrum-abc (script)
            |- setup.py
            |- electroncash -- __init__.py .
            |               |- ...
            |
            |- electroncash_gui -- __init__.py
            |                   |- ...
            |
            |- electroncash_plugins -- __init__.py
            |                       |- ...
            |
            |- icons/

Various resources and auto-generated files are placed in various places in that structure and outside (contrib...). The requirements*.txt files are in the contrib directory.

The structure I would like to have:

ElectrumABC -- setup.py
            |- bootstrap.py   (run the script in development mode)
            |- requirements.txt
            |- electrumabc |- __init.__.py 
                           |- __main__.py  (with code previously in the script)
                           |- tests |- __init__.py
                           |        |- ...
                           |
                           |- core -- __init__.py .
                           |       |- ...
                           |
                           |- gui -- __init__.py
                           |      |- ...
                           |
                           |- plugins --__init__.py
                           |          |- ...
                           |- resources -- __init__.py
                                        |- icons/
                                        |- fonts/
                                        |- protobuf/
                                        |- misc/    (servers.json...)

The resources subpackage would replace the current use of the Qt resource system. See https://github.com/silx-kit/silx/blob/master/silx/resources/ for an implementation.

The setup.py file would install the executable script as an entry point: https://chriswarrick.com/blog/2014/09/15/python-apps-the-right-way-entry_points-and-scripts/

This would be a first step to make the entire program installable using a simple pip install electrumabc command. Other steps include providing proper packages for some dependencies (secp256k1) as binary wheels for windows and MacOS.