Page support for "paginate = true" in view
We should add a "number of pages" property to the view, as well as a "current page" value, so we can show this instead of chapters in gnome-books' ePub view.
Whenever the view is loaded or changes size, we should:
- calculate the number of pages in each chapter, including the current one -> "number of pages" property
- add the length of each chapter before this one, plus chapter_length * position -> "current page"
It takes a while to recalculate all this, but iBooks on macOS/iPad pops up a window while it's recalculating, and ebook readers like the Kindle or Kobo will just "hang" when you change the font size for example.
Making sure to avoid blocking I/O, the view could send the number of pages/current page properties as being "-1" so the UI can show "calculating..." where appropriate, and disable the scrub bar, and then update the scrollbar and labels when it's done.