Chaka

• Based on MuPDF Android Viewer
• Add support of vertical page flipping
• Add support of text direction of right to left.
• Add support of single column mode. It will split a double-page spread image of a scanned pdf book into two seperated pages, except the first and the last page.
• Add current selection indication in fontsize interface.
• Rewrite Table of Contents, support collapsing and expanding of headings.
• Improve navigation and visual experience.