Add selection and shuffling support sections in QTI editor and runtime

In the QTI editor we support selection and shuffling of items within a section. The GUI configuration is transformed in the XML to a selection_ordering element. According to the QTI spec, this selection_ordering can also be configured for the assessment element, not only within the section element. Likewise, it should then be applied to the selection of sections and not the question items.

Example selection orderings:

      <selection_ordering>
        <selection/>
        <order order_type="Sequential"/>
      </selection_ordering>

     <selection_ordering>
        <selection>
          <selection_number>2</selection_number>
        </selection>
        <order order_type="Random"/>
      </selection_ordering>

Todo:

• Change runtime to read and implement selection_ordering in assessment elements
• Change editor to read and write selection_ordering in assessment elements
• Default is "select all sections" and "fix order"

Special care:

• Make sure default is applied when selection_ordering is missing
• Make sure already started tests without selection_ordering can be resumed properly
• Make sure existing edit sessions without selection_ordering can be resumed properly