Add language item for product multiplexing

Product multiplexing is currently enabled and configured via module properties in the qbs module. Unfortunately this comes with restrictions:

• multiplexed properties are static and pollute the qbs module
• early evaluation of the qbs module complicates the implementation of module loader

Proposal: Introduce a language item to control multiplexing
Requirements:

• allow multiplexing of arbitrary product properties
• allow multiplexing of arbitrary module properties
• allow profile multiplexing
• allow products to be multiplexed by multiple matrices

How to match multiplexed dependencies is a different topic and should be handled separately, refer to QBS-995 for instance.

Suggestion, syntax similar to Properties or Profile item

// Matrix with 2 properties multiplexed on 1 axis
Multiplex {
    // 2 properties multiplexed on a single axis
    qbs.architecture: ["avr", "msp430", "arm"]
    product.someProp: "someValue" // value is automatically expanded to match
                                  // the length of qbs.architecture
   
    // Matrix inheriting the outer matrix, contributing 
    // 1 additional property on a second axis.
    Multiplex {
        name: "matrix" // allows products to refer to it
        // 1 property multiplexed on a single axis
        product.anotherProp: ["yes", "no", "maybe"]

        excludedCells: [
            { 
                "qbs.architecture": "msp430", 
                "product.anotherProp": "no" 
            }
        ]
    }
}

The complexity grows from outer to inner. This is debatable, but it makes it very easy and elegant to define multiple matrices with a shared axis.

Structure: Multiplex wraps products:

Multiplex {
    Product {
    }
}

Structure: Multiplex in Product like Properties item:

Product {
    Multiplex {
    }
}

Structure: Matrix on the same level as Product items:

Multiplex {
    name: "matrix"
}
Product {
    multiplexedBy: ["matrix", ...]
}
// ...

Question: Would it make sense to let this item multiplex any other item?