nics: Improve dialogs with new framework components

 - Support in DialogTextInput for disabling, use it for "MAC".

 - Added DialogCheckbox, use it for "Persistence".  (Maybe also
   DialogCheckboxSet?)

 - Added DialogFormSelect, use it for "Source".

 - Added DialogRadioSelect, use it for "Type" and "Mode"

 - Change TypeAndSource to disable types without sources instead of
   disabling the sources selector.

 - When editing a interface, always include the current source in the
   choices, even if it doesn't actually exist.

 - When adding, re-create "default" network when none other exists to
   always have a path through the dialog.

 - Establish the "FooValue, init_Foo, Foo, validate_Foo" pattern.

Author: mvollmer