make this.own() handle Promises

[bill]

If this.own() is called with a Promise, it should cancel() the Promise when the widget is destroyed.

It should also forget about the Promise if/when the Promise is resolved or rejected. It should detect when the Promise is resolved/rejected by using Promise.then(), rather than using dojo/aspect.

Actually using always() would be slightly simpler than using then(), but always() doesn't seem to be supported by the Promise spec (see for example ​https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise), so it's not very forward-compatible.

[bill]

Actually, this was already reported in #17680... but the fix there needs to be improved so that Destroyable forgets about the Promise when it's resolved or rejected.