Add a File/Close option to the card manager.

* src/cardman.c (file_close): New.
(cardman_action_new): Add menu item.

All the other windows have a File|Close option, but the card manager
only has File|Quit. As a result, a user who tries to close the card
manager from the menubar will most likely shut down all of GPA, which
may not be their intent.