PDF allows for named destinations to be provided as string. These can be
either found in the Dests dictionary in the document catalogue (as
already implemented), or in the Name Tree specified by the Dests key in
the Names dictionary of the document catalogue (missing).
This commit adds this missing case. Once the named destination is found
in the name tree, its value is interpreted just like in the first case,
so a new utility method encapsulates the common behavior.
a5620fd41f