Loosely bound mode for Binding that allows multiple properties to be bound

With generalized grouped properties, you can do the following with Binding now:

Binding {
    testItem.width: 100
    testItem.height: 200
}

However, if the target is not statically known, you can't do that. We should allow a loosely bound mode like we have for Connections:

Binding {
    target: some.expression
    property int aPropertyOnTarget: 25
    property string someOtherPropertyOnTarget: "foo"
}

The original suggestion was as follows, but that requires custom parsers and is not type safe:

Bindings {
    target: testItem
    ignoreUnknownProperties: false
    width: 100
    height: 200
}