Improve handling of deleting an item type or item if they are required for/assigned to a project

[RobinGuth]

Item types & items being deletable might cause problems if they are already required for/assigned to a project

Ideas:

Items

• items should not be deletable if they are assigned to a project (but can be archieved)
• items should be deletable as long as the project is not completed
• more complex logic: for example items call always be deleted, but will require a replacement

ItemTypes

• item types should not be deletable if they or their any of their items are assigned to a project (maybe archive instead?)