register swipe diagonal gesture range to screenshot

for non-multitouch devices

This PR is intended to substitude #652 which needs tweaking
double tap zone for screenshot.

frontend/apps/reader/modules/readerpaging.lua

@@ -290,7 +290,6 @@ function ReaderPaging:onSwipe(arg, ges)
     else
         UIManager.full_refresh = true
     end
-    return true
 end
 
 function ReaderPaging:onPan(arg, ges)