This allows marking a MenuItem as a default action, e.g. in a
context menu for an action that reflects what e.g. a double click
would perform.
Also enhance the window menu to mark the close action as the
default, and when double clicked perform that action.
Fixes#1289
It didn't feel right to have a "DHCPClient" in a "Servers" directory.
Rename this to Services to better reflect the type of programs we'll
be putting in there.
