add regional zoom mode in pdf/djvu page
In regional zoom mode double tap will zoom to the tapped
region(paragraph or column, etc., detected optically via libk2pdfopt).
As the first demo, this feature is only turned on in flipping mode by
tapping the top-left corner of the screen. Eventually we may incorporate
this feature in "free" zoom mode.
Merge pull request #427 from chrox/master

add regional zoom mode in pdf/djvu page