tpl/collections: Improve type handling in collections.Slice

Fixes #5188

common/collections/collections.go

@@ -19,3 +19,10 @@ package collections
 type Grouper interface {
 	Group(key interface{}, items interface{}) (interface{}, error)
 }
+
+// Slicer definse a very generic way to create a typed slice. This is used
+// in collections.Slice template func to get types such as Pages, PageGroups etc.
+// instead of the less useful []interface{}.
+type Slicer interface {
+	Slice(items []interface{}) (interface{}, error)
+}