Support qxOwner property declaratively

[cboulanger]

In order to be able to declare the qx object id hierarchy via the qxOwner property, implement a new attribute"ownerId" which is translated into code like so:

if (qx,core.Environment.get("module.objectId")) {
    let ownerObj = qx.core.Id.getQxObject(ownerId);
   // omitted: assert that owner object exists and component has an id
    ownerObj.addOwnedQxObject(this, this.getQxObjectId());
// ...

This way, hierarchies can be declared that deviate from the parent-child relationship of the widgets (the reason why it is called "owner" and not "parent").

[gnikolaidis]

@cboulanger is the feature request still relevant? If so, we will implement it for upcoming 0.8.