Properties defined in the app are not shown in code completion

This is split off from https://github.com/ArcGIS/appstudio-vscode/issues/53

In the following example, it would be good if the property `primaryColor` appeared in the code completion list for `app.`

Qt Creator does do this.

```
App {
id: app
width: 400
height: 640

property var primaryColor: "blue"

Rectangle {
anchors.fill: parent
color: app.
}

}
```