Provide self-installing distribution packages

[peter88213]

Provide distribution packages for novelibre and its plugins/tools created with zipapp.

Instead of .zip files with setup.py scripts included, provide .pyzw packages. These are zip files which are executed by the Python interpreter, launching the included __main__.py script. This makes installation more convenient.

• Replace setup.py with __main__.py.
• In __main__.py, replace copy commands with extract commands.
• Have the Ant build script use zipapp instead of its built-in packer.
• Remove optional add-ons, or have __main__.py extract them.
• Update the documentation.