Offering useful parent property completions

It would be great to present sensible completions for an empty parent property:

e.g. here we could show the following list:
* All possible parent candidates (resources of the right type) in the template
* Offer to generate and insert an 'existing' parent for you
* Offer to generate a resource parameter (once #2246 is done)

If we could build this logic for non-snippet cases, then I think snippets would also benefit from it (we could leave it blank with a tab-stop, and pop up the completions).