bringing back the lost per-project cmake executable setting

[René J.V. Bertin]

Another thought: it's too early to start replacing the "CMake Binary" key in project settings files. For at least as long as 5.2.0 isn't out and readily available in the main distributions people should be able to go back to using 5.1.2 (or whatever they have installed) after testing a development build.

I suppose that ideally we'd be saving both the old and new keys for a reasonable transition period.

R.