Search for things/features with pending "desired" properties

[thjaeckle]

As an enhancement of #696 it should be possible via the Ditto search to find out which things have pending (not yet applied) "desiredProperties".

The syntax for that could be:

exists(features/*/desiredProperties)     // if we also delete an empty "desiredProperties" when all desired properties were cleared
exists(features/*/desiredProperties/*)

# or introducing a new RQL keyword for checking if a JSON value (object, array, string?) is empty:
empty(features/*/desiredProperties)
not(empty(features/*/desiredProperties))

• define the RQL syntax
• implement the query to do against the search index to support that

[jufickel-b]

I favor the "exists" approach. If a feature has no desired state then there shouldn't be a "desired" field in the JSON object – even if it is empty. I. e. as soon as the desired state is reached the "desired" field should be deleted.