Ignore LayoutItemPortals in the details view.

Ignore LayoutItemPortals in the details view.

I added a new DTO for the LayoutItemPortal so that I can ignore it in
the client code.

* src/main/java/org/glom/web/client/ui/DetailsViewImpl.java: Ignore
LayoutItemPortal layout objects.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Create
LayoutItemPortal objects when retrieving the details layout.
* src/main/java/org/glom/web/shared/layout/LayoutItemPortal.java: New
file. This is an empty class and just used to get type information for
now.