Bookmark flipping mode is toggled when long-holding the upper-right
corner of the screen. In this mood both the flipping icon and the dogear
icon are shown. And swiping west and east in this mode will flipping
only on bookmarked pages. Currently only pdf/djvu readers are supported.
This should implement feature request in #154.
Otherwise if text widgets in toc window expands out of screen, memory
free on these widgets will mess up the heap. It sounds ridiculous but
after we restrict toc window well inside the screen the bug in #815
is fixed.