Allow managing srv:serviceStandard elements in Manage Directory

[alcinnz]

It'd be great if we only needed to write the name of the standard and have all the subfields populated correctly!

[alcinnz]

Fixed in 4779a3140d109745bb5c93f78f80c288b1886cb0 by editing:

• $schemaplugin/layout/config-editor.xml to include a <for name="srv:serviceStandard" addDirective="data-gn-directory-entry-selector"> element, telling it to show a dropdown selector of directory entries for populating srv:serviceStandard.
• $schemaplugin/update-fixed-info-subtemplate.xsl to accept the srv: namespace in the match="/root" xsl:template.
• $schemaplugin/index-fields/index-subtemplate.xsl to include a xsl:template for extracting the title from a srv:serviceStandard. See commit for details.
• $schemaplugin/schema-ident.xml needs the element added directly inside it's <elements type="root"> element.