Signature should add reference to current task

[tjololo]

This issue has been investigated and is ready for implementation via the following PRs:

• Add task reference to signatures [part 1/2] #392
• Add task reference to signatures [part 2/2] #404
• Delete signatures if task process is reversed (rejected) app-lib-dotnet#617

Description

DataElement can add references.
When Storage generates a signature and stores it in a DataElement it should add a reference with the values:
Value: Intstance.Process.CurrentTask.ElementId
Relation: RelationType.GeneratedFrom
ValueType: ReferenceType.Task

This will ensure that if a user navigates back/forward old signature objects will be deleted when a signature task starts