#7 add support for non-sequential navigation

[Stephan Lehmke]

So far, the pages of a document are arranged
strictly in sequence. This is clear for
the usual sequence of pages or `slides', but
also for pages which are displayed incrementally.

Navigation only allows to move forward/backward
within the sequence.

Using links and pdf actions, it is possible
in principle to provide a more `tree-like'
arrangement of pages.

There should be support for at least the following
constructs:

* pop-ups, i.e. boxes which appear at a certain
position on the screen, hiding other parts currently
visible, and contain a further explanation of
some concept, info about a person mentioned etc.
They are activated by clicking on a link or
button and can themselves be displayed incrementally/
contain firther popups.
There should be a button for making the popup
vanish and return to the `main page'.

* excursions, i.e. whole parts of the document
which are not part of the main sequence, but
are only reached by a link or button. They can
themselves be incremental or contain further
excursions. There should be an easy way for
returning to the point where the excursion was
taken.

* variants, for instance a version of the same
document with a specific color/font selection
for the vision impaired, or a `view' of a
presentation where certain uninteresting
parts are left out. Navigation helpers should
allow to switch variants at any point within the
presentation.

* buttons to skip or rewind an `animation' created
with \multistep or \movie.

It needs to be researched in how far pdf
actions, xforms or annotations can help here.