By porting common triggers to actions, dialog buttons to action widgets, GtkMenu to GtkPopoverMenu, using the g_rc_box API, and add braces everywhere. These will mostly make porting to GTK4 easier.
Also, added a default Service name of the JID. Otherwise, looking at xmpp.org, you previously got a big list of empty-looking rows (which also warn when the tooltip is shown).

Opened plugin window, Browsed to xmpp.org and pidgin.im, checked that the Add button opened the Add-to-chat dialog, checked that double-click did the same, checked that the menu item did the same. I did not check Register in any of its forms as I don't know what server to query that returns something which could be registered on.