[UX] Gesture manager: add default previous/next chapter gesture

* Up, down, up for previous.
* Dow, up, down for next.

frontend/apps/reader/modules/readergesture.lua

@@ -78,6 +78,8 @@ local default_multiswipes = {
     "west north",
     "east south",
     "west south",
+    "north south north",
+    "south north south",
     "west east west",
     "east north west",
     "south east north",
@@ -109,6 +111,8 @@ function ReaderGesture:init()
         multiswipe_south_east = "toggle_reflow",
         multiswipe_south_west = "show_frontlight_dialog",
         multiswipe_west_south = "back",
+        multiswipe_north_south_north = "prev_chapter",
+        multiswipe_south_north_south = "next_chapter",
         multiswipe_west_east_west = "open_previous_document",
         multiswipe_east_north_west = "zoom_contentwidth",
         multiswipe_south_east_north = "zoom_contentheight",