NetworkSetup: put interface settings in main window

* The initial design was probably inspired by Windows network settings,
which are a mess of nested windows and hyperlinks.
* Instead, the settings can be put directly inside the main window, like
it is done inthe media preflet. This makes the information available at
a glance and makes the UI easier to navigate.
* Remove the "configure" button and popup menu item. A single click is
now enough to show the settings for an interface

For this to work properly, complete the move to layout kit (there were a
few BRect-based constructorls left which would mess the layout) and
remove unneeded rectangle computations (layout kit does the job
already).

Further cleanup pending:
* Removal of the duplicate "revert" button
* Removal of the "save" button and switch to applying settings realtime
* Move the disable/enable and renegociate buttons inside the interface
settings
* Remove the top-level tab view (nested tab views are not nice) and
use a hierarchized list with both interfaces and services on the side
(like the Media preflet has audio and video).