keyboard: make Add button insensitive after editing

When creating a new keyboard shortcut, the Add button gets
sensitive when all the fields are valid. If the user tries
to create a new shortcut right after closing the shortcut
editor dialog, the Add button is still sensitive even with
invalid custom fields.

Fix that by making the Add button sensitive whenever we
finish editing the new custom shortcut.

https://bugzilla.gnome.org/show_bug.cgi?id=769314