Emacs Xref improvements

[mattiase]

Fix uses of Xref from mouse events, such as the context menu new in Emacs 28, by implementing the necessary methods and smuggle the buffer position in a text property of the string.

We also get better highlighting of Xref hits in the target buffer by taking advantage of a bug fixed in Emacs 28.

[mattiase]

With the last part of this PR, xref can now more easily be used for looking up definitions and occurrences of operators.

[voodoos]

@rgrinberg I requested your review since you appear to be the author of the initial xref support.

[rgrinberg]

I don't use any Emacs anymore and don't remember much of elisp. I think we need to find someone else to review unfortunately.

[mattiase]

Tell me if there is anything more I should do with the patch set. I did consult the author of the Xref facility in Emacs for how best to use the interface in a way that's compatible with different versions.

[bbatsov]

The changes look good to me.

[mattiase]

Not sure what this patch is waiting for – all rather straightforward things I thought. Anyway, it's up to date with master.

[bbatsov]

@rgrinberg Can you help us to get this merged?

[voodoos]

Thanks for your review @bbatsov, and @mattiase for rebasing the PR. Sorry it took so long.

@mattiase could you add a changelog entry for this one ? Thank you !

[mattiase]

@mattiase could you add a changelog entry for this one ?

Done.