Add validator and workflowitem type field to subproject

[Stezido]

Description

Currently there is no possibility to make sure only one user is able to close worklfowItems.

Solution

When creating a workflowitem the assignee should be selectable (default: creator).
When creating a subproject a fixed workflowitem type should be selectable (default: any)
When creating a subproject a validator should be selectable (default: none)
Validator="A user(no group) who is preselected at workflowitem creation (not selectable). This user is the only user who can close workflowitems."