Correct library directory for zip-files

Let's keep our fingers crossed that this doesn't break anything.
New sd xpath function: current-framenumber()

This function returns the current frame in the given positioning area.
Building nested Element structure fixed

When building nested Elements in a variable such as

<SetVariable variable="foo">
   <Copy-of select="$foo" />
   <Element name="sub">
     ...
   </Element>
</SetVariable>

and using this structure inside another Element like this

<Element name="whatever">
  <Copy-of select="$foo" />
</Element>

we have an intermediate table between the outer Element
and the inner Element. Which breaks when trying to save
the structure to disc.

When building the Element structure we now explicitly look
for the intermediate table and remove it.
Better error messages
Box bleed="left" as the fourth option