Prototype C++ and QML interaction

QML compiler relies on the runtime infrastructure to evaluate functions (script bindings, JavaScript code, etc.). We should figure a convenient way of interaction between generated code and this runtime. So far, an approach of "engine run this function by index" seems to work fine, but tests are needed. Specifically, test that:

• basic lazy QML bindings work. E.g.

  width: height + 1 // if height changes, width should change as well
  

• (C++ generated) signal handlers can pass input/output arguments to the engine and these arguments are usable. at the same time, other variables in scope (e.g. properties of object) are also usable

  signal signal(string x)
  onSignal: function(x) {  // defined in C++, calls the engine
    console.log("received: " + x);
  }
  

• property change handlers work

  onHeightChanged: {  // defined in C++, calls the engine
    console.log("new height: " + x);
  }
  

• (C++ generated) bindings can be reset from QML (e.g. as a result of some function invocation)

  width: height + 1  // initial binding, defined in C++ [1]
  onSomeEvent: {
    width = 10  // no binding [2]
    // and:
    width = Qt.binding(height * 2); // new binding [3]
  }
  
  // in C++:
  "getWidth()"
  [1]: returns height + 1 (for some value of height, changing height changes width)
  [2]: returns 10 (and changes to height do not change width)
  [3]: returns height * 2 (for some value of height, changing height changes width)
  

• QML bindings work on C++ generated properties (related to previous point)
• runtime code evaluation behaves correctly when JavaScript functions are involved (e.g. property lookup works inside JS
• property aliases work

all of the above should also apply to subobjects

QtObject {  // root object
  QtObject {  // subobject
  }
}

... and to property aliases