Update maindialog.py

Reorganized find_services method to eliminate duplicating listings of acceptable link types. To make it easier to manage those listings.
Update maindialog.py

Added to list of acceptable link types, in find_services method, the urn protocol references strings that seem to be the convention in GeoPortal. eg: 'urn:x-esri:specification:ServiceType:Gmd:URL.wms'
Merge pull request #43 from emiliom/master

Recognize GeoPortal-style service/link type code types (WMS, WFS, WCS)