Lambda is not an API, but it connects to a signal that's defined in the header and should match up function signatures. I wouldn't say that having const is required for making this function/lambda correct.
If you feel strongly about it, have the const. I'll still want to figure out how to consistently declare my own functions (one way or the other), but I'll stop asking for removing const from other people's patches. It's not important enough for me to continue arguing.
And the signal above definitely should be declared with const QString & instead of plain QString. Presumably this was introduced without the knowledge that moc handles these just fine and doesn't need by-value parameters. But it was already in place before Fabian's patch, so it falls outside the scope of things to ask for in !370 (merged) directly.