Use `expand` on a value set from a target selector

Reading and participating in this discussion made me realize how challenging it is to use an input or parameter set using a target selector in a template. It works great if you want to just use it directly in a service call but if you want to filter some of the entities out things get difficult quickly.

It would be great if expand worked on this object. So you could simply expand it into a list of entities like a group and not have to separately deal with entities, devices and areas in the same complex template.