Rules for items with multiple revisions

[odungern]

In branch 'master' for v0.11.x and later, an item may have multiple revisions.

For items, the following rules apply:

• If there are items with the same id, they all must specify a revision.
• All revisions of the same item must be different (the key consisting of id and revision must be unique).
• All revisions of any item must be >0.

For references (such as 'subject' or 'object' in statements), the following rules apply:

• If the reference does not specify a revision, the latest revision of the item is taken.
• If the reference specifies a revision=0, the latest revision of the item is taken (thus a revision=0 may only be found in a reference).
• The item referenced by id and revision must be present in the project or file.

ToDo: Upgrade the constraint check to enforce these rules.

[odungern]

To have a general approach, a revision must be a GUID, now, and not just an incremented number. So the rules with respect to '0' don't make sense. Thus, the rules are modified.

For items, the following rules apply:

• If there are items with the same id, they all must specify a revision.
• All revisions of the same item must be different (the key consisting of id and revision must be unique).

For references (such as 'subject' or 'object' in statements), the following rules apply:

• If the reference does not specify a revision, the latest revision of the item is taken. The latest revision is the one which is not replaced by another.
• The item referenced by id and revision must be present in the project or file.

The constraint check does enforce these rules, so the issue is closed.