Support Xsl 2.0 elements

[frederikbosch]

Looking for source with all xsl 2.0 elements.

• [x] xsl:function
• [x] xsl:for-each-group group-by
• [ ] xsl:for-each-group group-adjacent
• [ ] xsl:for-each-group group-starting-with
• [ ] xsl:for-each-group group-ending-with
• [x] xsl:value-of: separator and sequences (BC break with XSL 1.0)
• [x] xsl:attribute with select and separator
• [ ] xsl:perform-sort
• [ ] xsl:sequence