When loading an alternate configuration use that file as
current configuration filename.
Make the filename visible in the dialog.
Default continue to be .config.

Inspired by patch from: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc:  Cyrill Gorcunov <gorcunov@gmail.com>