Allow saving/restoring layout of key list to/from custom config group

KeyTreeView:
* Make KConfigGroup argument mandatory (to prevent wrong usage).
* Add config group argument to saveLayout() and restoreLayout() and
  make both methods public.
* Automatically save/restore layout only if config group was passed
  to constructor.

DeleteCertificatesDialog, NetKeyWidget:
* Explicitly, save/restore layout of key list(s) to/from custom config
  groups.  Previously, the layout was saved to/restored from the config
  group "KeyTreeView_default" implicitly.

TabWidget:
* Explicitly, use config group "KeyTreeView_default" for temporary
  key lists (e.g. after importing certificates). Previously, this
  config group was used implicitly.

GnuPG-bug-id: 5175