bugfix: use pageno from listened events other than the page in view state
As the view state may not be updated yet when PageUpdate event is triggered.
add a dog-ear-like icon at the above right corner of bookmarked page
And tap the above right corner will toggle the bookmark on this page on/off.
set page background color to white in pdf/djvu reader
do full refresh every six page turning/rolling
Merge pull request #768 from chrox/dogear_bookmark

show a dog-ear-like icon on bookmarked page