Enhance OM: Global Exclude via ItemDefinitionGroup

Parent issue: #700

Goal: Enable a way to specify a global exclude pattern to be applied to all items of a certain type. The alternative is to define a Property containing the global exclude patern and then append it to all the excludes of all the tags of a certain item type.

Design so far:

``` xml

```

Behaviour for the definition group exclude attribute:
- When multiple ItemDefinitionGroups for the same item type have excludes
- latest wins. If it is additive, the user cannot overwrite it.
- What can be referenced from the exclude:
- properties
- metadata from ItemDefinitionGroup items of this type
- When an ItemGroup item has its own exclude
- union the local exclude with the global exclude

Build script pitfalls:
- With this implementation, Compile won't ever be able to refer to something in Obj. `CoreCompile & friends` need to grab generated code from Obj via a different `TransientCompile` item