Export icons according to XDG icon spec on unix

[matterhorn103]

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[matterhorn103]

This means we will then be following the XDG icon spec, as practised on all modern Linux distros and as required by flatpak-builder-lint.

[ghutchis]

Can you remove the clang-format commits from this? I'd rather do that separately. It looks like you're using a different version of clang-format.

[matterhorn103]

Done.

I had applied the diff that the automatic clang-format check produced, so which version I have shouldn't matter - it should have been in agreement with whichever version of it the GitHub Action uses. But it's no matter, I hadn't changed any C++ files anyway.