Add support sub-group in the details view.

Add support sub-group in the details view.

I also removed the code that special-cased the default details view
layout. See:

http://mail.gnome.org/archives/glom-devel-list/2011-July/msg00005.html

I still have to make a proper flowtable.

* src/main/java/org/glom/web/client/activity/DetailsActivity.java:
Don't special-case default details view layout.
* src/main/java/org/glom/web/client/ui/DetailsView.java: Remove
addLayoutField() as I'm going to use it.
* src/main/java/org/glom/web/client/ui/DetailsViewImpl.java: Extract
GroupPanel to FlowTable class. Remove unimplemented addLayoutField()
method.
* src/main/java/org/glom/web/client/ui/FlowTable.java: New class
extracted from DetailsViewImpl.GroupPanel. Add support for
sub-groups.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Set
column count when getting the details layout.