add flipping mode in pdf/djvu reader

By tapping on the upper left corner of the reader one can toggling flipping mode on/off.
In flipping mode swiping right/down and left/up  will paging backward and forward
accordingly with pages proportional to swiping distance.

frontend/ui/reader/readermenu.lua

@@ -21,8 +21,9 @@ function ReaderMenu:initGesListener()
 			GestureRange:new{
 				ges = "tap",
 				range = Geom:new{
-					x = 0, y = 0,
-					w = Screen:getWidth()*7/8,
+					x = Screen:getWidth()/8,
+					y = 0,
+					w = Screen:getWidth()*3/4,
 					h = Screen:getHeight()/4,
 				}
 			}