Variable queries support for Tasks

Split as requested from https://capacities.io/feedback/p/support-variable-queries-and-collections-in-tasks

Variable/embedded queries aren't supported for Task properties, e.g.

• Due date is within the range of the date property of the on containing object (only the basic before/after today/week/month available)
• Task Tags include tags (or a tag-based property) in the containing object