Support wildcard item selection for Union layers

[keithmoss]

Currently we need to manually specify the extra attributes we want Union layers to support (e.g. in GetFeatureInfo requests) by using the ITEMS processing option. e.g. ITEMS=field1,field2,...,fieldn

It would be great if, similar to how the "gml_include_items" metadata property works now, we could specify "ITEMS=*" and get all fields from the source layers.