QScxmlEcmaScriptDataModel: Allow different state machines to share a single QJSEngine

When using the ECMAScript data model, each state machine instantiates its own QJSEngine. Consequently, each machine takes up 2 MB of GC heap and 4 MB of stack space upon initialization.

It would benefit resource-constrained systems if the different state machines in a process can share the same QJSEngine.

Currently, state machines store their state data directly in the engine's global object's properties. Perhaps each state machine could add a single "data object" to the global object, and store state data in the data object's properties instead?