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

Added additional acceptable link type strings for WMS, WFS, WCS, based on urn code types commonly used in GeoPortal (eg,  'urn:x-esri:specification:ServiceType:Gmd:URL.wms'). All changes limited to plugin/MetaSearch/dialogs/maindialog.py, to MetaSearchDialog find_services method.

Note that I haven't tested this in QGIS! I've never done a pip/git install that replaces an installed QGIS plugin, so I'm hesitant to do it right now. But the edits are pretty simple.