Avoid binding allocations for side-effect free property initializers

We can observe that QML files are sometimes used to store settings / styling type of information. It acts like a JSON file but it can be used directly in bindings and is human readable and editable.

For example Placements.qml:

import QtQml 2.0
QtObject {
    readonly property rect homeButtonPlacement: Qt.rect(0, 0, 20, 20)
    readonly property rect backButtonPlacement: Qt.rect(100, 100, 20, 20)
    ...
    readonly property var defaultColors: {
        "homeButton": "green",
        "backButton": "black",
    }
    ...
}

Often code like that is generated from an external source.

At the moment when the QML engine loads such a QML file, we evaluate JS binding expressions for the Qt.rect(...) calls as well as for the object literal expressions.

While there exists the option of considering making these two constructs built-in and detected at compile time, another more general optimization surfaces:

If a binding expression does not depend on any other properties that might change (in engine terms: there are no captures registered after evaluation) and the evaluation result is assigned to a read-only property, then we can conclude that afterwards the value cannot change anymore. Therefore we can remove the binding object that was used for the initialization.

This saves memory.

On a large scale application this optimization saved up to 5% of all bindings on start-up (~9000 of ~180000). On Linux x86-64 one binding is approximately 144 bytes, so the savings are in the range of ~1.2 MB of heap, as well as reduced fragmentation.