Smart defaults for new connection in query tool

[brsa]

On opening the "Connect to server" dialog, "Database" and "Username" fields are copied from the current connection. So far so good.

After choosing another "Server", "Database" and "Username" fields default to the alphabetically first available there. It would be mighty convenient, however, if the tool could pick identical database and/or user in case those are present on the new server. Only failing that fall back to the alphabetically first.

This was part of #209, but got lost in the details.
Also, somewhat related to #211 and maybe #231.

[gleu]

Patch at  http://github.com/gleu/pgadmin3/commit/8c358a7ead668bc7b2f7c845ab83b4b3c8f4d1aa