Variables: Support for object values / multiple value properties

[torkelo]

I explored variables with many "value properties" first in #995 . The thing that troubled me is that the natural continuation for that is to support it for Query variables as well, and from a user perspective building a dashboard it becomes very muddy/confusing to introduce basically identical variable types with the only difference being one support values with many properties.

The same problem exists for #1228 when we want to introduce it to dashboards (and build on it to support object values for variable queries)

In this PR I am taking the first step in supporting "object" / multi property values in MultiValueVariable. Which required a very small change.

Next I would explore

• What small change would be have to make to QueryVariable to now take advantage of "object" / multi prop variable values
• What small change would be have to make to CustomVariable to now take advantage of "object" / multi prop variable values, support json?

^ I am not doing this in the next week or so, anyone who has time could start exploring the above.

Open question

• Should we support "multi" select (ie, multiple different object values).

[grafakus]

Thank you for the PR @torkelo , I've continued the exploration in #1236