Line numbering for text frames

Here is an idea that comes from a feature request, specifically http://bugs.scribus.net/view.php?id=1975

As you can see this was filed back in 2005, so it's mostly been gathering dust, or whatever bug reports do over the years. Kunda wondered if this might be scriptable, so I began to see what I could do. Consider this a work in progress.

What the filer wanted was the ability to have numbering of lines present on a proof of some document, so that this numbering scheme could be used when one person is recommending an edit to be done by someone else in some particular frame on some particular page. Later, the numbering would eliminated or turned off for the final product. Kunda dug up this bug, and thought that it seemed an interesting concept, and might be useful for other purposes, purposes unknown as of yet.

linenumber.py Version 2
Now we add the code to create our numbering frame on its own layer, "numberinglayer". Since we may have already run the script, we need to check first if it already exists, and if not create it. In either case, we set this as the active layer.