Allow Probe items inside Project items

It could be useful to find and include external projects or products into the project.

E.g. I have an external source library which I want to include into the project. I can also want to update its sources sometimes (e.g. by calling rsync or git). The location of this library can vary for some reasons, so I have to check several paths to find it's location first.
To achieve this now I have to make such tests inside a property evaluation like

property path externalLibraryPath: {
    // check the location 1
    // check the location 2
    // update with rsync
    return path + "/library.qbs"
}

Then I can use this property in references to include the library.

But this kind of tests are usually performed by Probe items. Also probes have good caching mechanism which allows to control when they should be re-evaluated.

The only problem here probes are allowed only in products.