core: fix calls to `domish.Element.elements`: domish.Element.elements should be called with namespace and element name as 2 arguments, but it has been confused in several places with the call to `domish.Element.addElement` which is often done with a `(namespace, name)` tuple. Unfortunately calling with a tuple is accepted and doesn't raise any error in `elements`, but this result in a wrong element returned. This patch fixes the erroneous calls.

[Desktop Entry]
Type=Application
Name=Libervia TUI (Primitivus)
Comment=console frontend for Libervia (XMPP)
Exec=libervia-tui
Icon=org.libervia.LiberviaTUI
Categories=Network;InstantMessaging;Chat;FileTransfer;RemoteAccess;
Keywords=xmpp;chat;messaging;im;file;sharing;communication;jabber;social;network;decentralization;console;