Fix Staff &lt;barLineSpan&gt; file tag:

With the 'new' XML reader, tag attributes have to be read before reading the tag main value.

For Staff &lt;barLineSpan&gt; tag, this has not been adjust yet.
add svg icon engine
Merge pull request #355 from mgavioli/Fix_staff_barLineSpan_tag_reading

Fix reading Staff <barLineSpan> file tag from score