RFC: Modernizing configuration management

These are some hand-wavy ideas for improving Weston's configurability.

Current situation:

• configuration is read only once at Weston start-up, and cannot be changed at runtime
• weston.ini uses a home-grown parser, weston_config, that is not worthwhile extending

I would not limit us to be backward-compatible with current weston.ini nor even use an INI format at all if better alternatives exist. I'd suggest a new Weston frontend with the new configuration tools. The current frontend can be left with the current weston.ini to not break existing users.

Wishlist:

• ability to change configuration dynamically; read, modify, and write configuration files, so that dynamic changes can persist on next start-up
• use a library, so we do not need to develop or maintain the fundamentals
• complex data types, e.g. vectors/lists as values

Additionally, we would need an IPC for actually changing the configuration. If there is a library offering also that, e.g. via D-Bus, that would be cool. Then we could expose volatile debug settings too.

Not all settings would fit a generic configuration library, though, e.g. #341, unless the library offers testable transactions. This gets easily complicated, so perhaps we should keep the IPC separate from the config library. Some topics have prior art, like wlr-output-management for #341.

This is also orthogonal to alternative configuration sources like requested for in #865 which would replace parts of configuration rather than be an API to change and save it.