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: T5175