connections-pane: handle postgres materialized views

[detule]

Closes https://github.com/r-dbi/odbc/issues/255

Thought about going couple of different ways here - including masquerading the materialized views as simple "view" s. In the end opted for leaving them marked as they are and handling "matview" in the self reported "objtype" structure.

Pros/cons to both - happy to discuss and/or go in a different direction.