imprv: Simplify URI context menu
This MR improves URI context menu handling in Gajim by:
- removing custom URI handling for all types except XMPP/ambiguous URI types. GTK handles other URI types already (mailto, tel, etc.) in a cross-platform compatible way (works on Windows as well) (closes #12207 (closed))
- fixing #12192 (closed) by removing "ambiguous-address" URI processing
- removing "allow_open_file_uris" setting and URI handling (this is considered feature-creep)