Add unique attribute for Section Schema DSs.

Section Schemas Datasources should have an attribute `type` with value set to `section-schema` or something.

Use case:

I have several DSs attached to a page that return the schema of sections and I want to select a specific Datasource by section ID:

    <xsl:copy-of select="/data/*[ @id = $section_id ]"/>

All good, but what if another Datasource that is not Section Schema type will have an attribute `id` with value equal to `$section_id`? That would be a conflict and above code will generate undesired results.

In the spirit of defensive coding, I suggest merging the above commit, thus resulting in this XSLT code:

    <xsl:copy-of select="/data/*[ @type = 'section-schema' and @id = $section_id ]"/>

Future safe.