Add xkb_get_version()

[wismill]

Enable to apps to query the version of the libxkbcommon library actually in use and adapt their behavior: e.g. supported flags and available symbols, etc.

Fixes #932

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TODO:

• Better commit message
• Documentation with examples (do and don’t)
• Changelog
• Add a flag arg to enable future developments?

[wismill]

@bluetech @whot See #932 for the rationale. My main reluctance for this function is that it can be abused or the app workaround get out of sync, while testing failures could be a more robust solution (e.g. unsupported flags¹, etc.).

Footnotes

1. Note that xkb_context_new() currently does not fail on invalid flags. Probably worth fixing. ↩

[Caellian]

I will go with symbol testing in the end for what I need. I see xkb_get_version as something useful only if functions like xkb_keymap_is_supported_format, xkb_context_is_flag_supported, etc. are missing for functionality that can't be probed through symbols or require e.g. constructing context+keymap+state to check whether something is supported.

Wayland allows requesting specific protocol versions, so I guess this would simplify some cases the same way it helps with protocol versioning in WL. But I agree now that it's bad to rely solely on version to handle all version differences in the library because that requires the app/bindings/wrapper to stay on top of all the changes to this library.

There's potential for misuse, but it could also pad omissions and mistakes. I suggest you mention in the docs "prefer other means (e.g. checking symbols) instead of relying solely on this value if possible".