Enable retrieving "resolved" binding for a property in QQmlJSScope

Consider

Item {
   id: root
   anchors.left: foo.left
   anchors.top: bar.bottom
}

When analyzing root's anchor property, we often want to analyze the effective QQuickAnchors object with bindings to left and top, and not the separate grouped bindings. We should provide a method that returns the "end-result" which the engine creates after applying all grouped bindings.

An initial implementation must merge all grouped bindings in the same scope; further work will be needed to cover

• bindings set up in the "base" type with grouped bindings in the "derived" type,
• Immediate properties,
• bindings in different scopes,
• and (mostly resulting from the last point) conflicting grouped properties