This is a huge configuration refactoring.
The configuration system now uses Pydantic settings, with better validation, better
support for environment variables and other goodies.
The main config files now use TOML instead of INI, this allow for better handling of
complex types, notably dict.
Plugins, components and frontends can declare their config settings in a class which will
be imported dynamically.
``call`` commands now handle various output. Beside the original one (now named
``simple``), a new ``gui`` one display a full featured GUI (make with Qt).
PyQt 6 or more needs to be installed.
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This command has 2 subcommands: `make` and `receive` to make a new call or wait for one.
When call is in progress, a window will be created to show incoming stream and local
feedback, and a text UI is available to (un)mute audio or video, and hang up.
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