Structured log messages with a message registry

Structured log messages with a message registry

Currently there is little structure in the log messages, making
difficult to use them for the following use cases:

- A user looking for help about a log message: the user probably
  uses a search engine, thus the results will depend on the proper
  indexing of our documentation and the various forums. It relies
  only on the wording of the message, which may change with time.
- A user wants to filter the logs resulting of the use of one of the
  components of xkbcommon. A typical example would be testing
  xkeyboard-config against libxkbcommon. It requires the use of a
  pattern (simple words detection or regex). The issue is that the
  pattern may become silently out-of-sync with xkbcommon.

A common practice (e.g. in compilers) is to assign unique error codes
to reference theses messages, along with an error index for
documentation.

Thus this commit implements the following features:

- Create a message registry (message-registry.yaml) that defines the
  log messages produced by xkbcommon. This is a simple YAML file that
  provides, for each message:

  - A unique numeric code as a short identifier. It is used in the
    output message and thus can be easily be filtered to spot errors
    or searched in the internet. It must not change: if the
    semantics of message changes, it is better to introduce a new
    message for clarity.
  - A unique text identifier, meant for two uses:

    1. Generate constants dealing with log information in our code
       base.
    2. Generate human-friendly names for the documentation.

  - A type: currently warning or error. Used to prefix the constants
    (see hereinabove) and for basic classification in documentation.
  - A short description, used as concise and mandatory documentation.
  - An optionnal detailed description.
  - Optional examples, intended to help the user to fix issues
    themself.
  - Version of xkbcommon it was added. For old entries this often
    unknown, so they will default to 1.0.0.
  - Version of xkbcommon it was removed (optional)

  No entry should ever be deleted from this index, even if the message
  is not used anymore: it ensures we have unique identifiers along the
  history of xkbcommon, and that users can refer to the documentation
  even for older versions.

- Add the script update-message-registry.py to generate the following
  files:

  - messages.h: message code enumeration for the messages currently
    used in the code base. Currently a private API.
  - message.registry.md: the error index documentation page.

- Modify the logging functions to use structured messages. This is a
  work in progress.