[scribus] Keep master page logic when modifying page order

[ale rimoldi]

hi

> Scribus lack an event system which would signal when a page is
> visible or not.
> 
> With such an event system, we could bind some action into scripter
> and be much more effective at modifying the scribus behavior.
> 
> There is a lack for page visibility api as well. It is non sense to
> draw the whole document if we don't even see it. <-> BIG performance
> boost.

basically, you can also limit what gets updated when the content is so
small that it's not readable...

i think that you're very welcome to work in this issue!

just take care that it's a little bit more complicated than it first
looks like.

as an example, you will have to take care of text chains that go
backwards (and generally speaking of text that is written right to
left).

in a further step, you would probably also want to implement an update
queue and some multithreading so that the dirty pages get updated when
scribus is idle (if scribus thinks that it makes sense to do so)

finally, you might only want to trigger the optimization if the number
of pages in the document is n times (where n is 3?) the number of pages
fitting on screen.

and, yes, jluc suggestion is sane, to first provide a way to know which
items are on a page and on which pages items are.
i have a naive implementation of it, that can be improved for
performance and needs to be integrated with scribus in memory model an
SLA saving. (i use it for exporting, so performance is not that
critical)

have fun
a.l.e