Implement the flow table layout in the Details View.

Implement the flow table layout in the Details View.

* src/main/java/org/glom/web/client/ui/DetailsViewImpl.java: Change
FlowTable to Group to account for the renamed class.
* src/main/java/org/glom/web/client/ui/details/FlowTable.java: New
File. This is a container widget that implements the Glom details view
flow table behaviour.
* src/main/java/org/glom/web/client/ui/details/Group.java: Moved from
org/glom/web/client/ui/FlowTable.java.
* src/main/webapp/style.css: Adjust bottom margin of the subgroup-title
so that the size of the subgroups are a closer match to the size of
the Glom subgroups. This makes the flowtable layout match the layout
in Glom for the Music Collection example file.