split panes

Add split panes to the UI
aka "multiple panes", "split views"

Having the option to split the desktop workspace window
into multiple panes will allow for multiple views on
the document in work.
This can be used to look at a detail region of the
drawing while simultaneously seeing the whole picture
for purposes of better overview.
Another use case is to simultaneously work on different
drawings which may be stored in seperate parts of the
same svg file. This will require further inkscape work
to support svgs multipage feature or to add a
proprietary extension.
This can also be very useful for the people who use
inkscape to draw more than one 2D paths and then input
them to POV-Ray to go 3D from there.

If you would be interested in working on that, please
get in touch with <email address hidden>.

The issue was discussed in detail between
martin4inkscape and JonCruz on 06Nov14, discussion log
available as attachment.