add DSHOWOVERLAP in defaults.lua

Turn off "show overlap" by default since scroll mode can replace it
in most cases where "show overlap" is used. And the dimmed area often
make new users confused. It's better to leave this feature to more
advanced users.

frontend/ui/reader/readerpaging.lua

@@ -5,7 +5,7 @@ ReaderPaging = InputContainer:new{
 	number_of_pages = 0,
 	visible_area = nil,
 	page_area = nil,
-	show_overlap_enable = true,
+	show_overlap_enable = DSHOWOVERLAP,
 	overlap = scaleByDPI(20),
 	flip_steps = {0,1,2,5,10,20,50,100}
 }