Incident #24769: Add generic signal for scripting

I made this to scratch my own itch really, but think it will be useful to many. 

This can be used with the following example code:

```javascript
// connection and signal handler
mainwindow["emitSignal(QString, QString)"].connect(emitTest); 

function emitTest(source, message) {
  if (source == "myCustomScript") {
    QMessageBox.information(mywindow, source, message);
  }
}

// sample function for emitting the signal via script
function sendTestSignal() {
  mainwindow.sEmitSignal("myCustomScript", "my custom message");
}
```