Add main groups to the details view.

Add main groups to the details view.

This makes things look a little nicer in the details view. The next step
is to implement the flowtable.

* src/main/java/org/glom/web/OnlineGlom.gwt.xml: Inherit only the
resources from the standard gwt css theme. Standard.css is now
included in OnlineGlom.html so that the online glom css rules have
precedence over the gwt theme.
* src/main/java/org/glom/web/client/activity/DetailsActivity.java: Pass
the whole LayoutGroup to the DetailsView instead of just the titles.
* src/main/java/org/glom/web/client/ui/DetailsView.java:
* src/main/java/org/glom/web/client/ui/DetailsViewImpl.java: Create the
details layout with a helper class (GroupPanel). I might extract this
class when I make the full flowtable.
* src/main/java/org/glom/web/shared/layout/LayoutItem.java: Add empty
string as default so I don't have to worry about NPEs when processing
the layout objects.
* src/main/webapp/OnlineGlom.html: Add the gwt standard css file (see
note beside OnlineGlom.gwt.xml above).
* src/main/webapp/style.css: Add default font-size to body to override
the font-size set by the standard theme. Don't use h2 tags for
group-title. Create new details-cell class.