Refactor bookpage.cpp
This file contains a lot of things:
- BookPage class - a page widget itself
- PageDisplay, which is basically a formatted page.
- Typesetter, which is basically a set of styles.
- A lot of low-level formatting-related stuff (such as GlyphStream and RenderXform).
As result, that file has size about 1500 lines, and we can split it to several separate files.
Also classes names are not really descriptive, so it is a quite hard to distinguish BookPage from PageDisplay, or tell what the mRenderXform struct really does.