cleaning up output of markings

markings now show the narrative controlled structure in brackets after the
label for the marking structure type.

the xslt templates related to Marking have been cleaned up and the layout is
now more logical.

supports issue #58
showing marking controlled structure on TLP

the previous commit meant that the marking controlled structure was not showing
on TLP markings (since they do not have a child element below
Marking_Structure).  This commit fixes that.

supports issue #58