connections: Use custom realname completion

EntryCompletion is tied to the Entry widget, so our realname
completion is currently preventing us from moving to libadwaita's
new EntryRow widget.

Luckily we don't use most of EntryCompletion's functionality: We
only have a single completion that is inserted directly into the
entry.

That use is simple enough to reimplement it ourselves.

Part-of: <https://gitlab.gnome.org/GNOME/polari/-/merge_requests/258>