Menu: Split off "Page Turns" submenu in two sections

The Kindle swipe animations & physical key inversion apply *everywhere*,
so we need this accessible in the FM, too.

Move it to Navigation, which is where it already was on !Touch devices.

frontend/apps/filemanager/filemanagermenu.lua

@@ -464,6 +464,9 @@ To:
         self.menu_items[id] = common_setting
     end
 
+    -- settings > Navigation; this mostly concerns platform-specific features or physical keys, and applies *everywhere*
+    self.menu_items.platform_navigation_features = require("ui/elements/platform_navigation")
+
     -- settings tab - Document submenu
     self.menu_items.document_metadata_location_move = {
         text = _("Move book metadata"),

frontend/apps/reader/modules/readermenu.lua

@@ -237,11 +237,11 @@ function ReaderMenu:setUpdateItemTable()
     end
 
     if Device:isTouchDevice() then
+        -- Mostly concern about touch related page turn stuff, which only applies to Reader; ends up in Taps & Gestures
         self.menu_items.page_turns = require("ui/elements/page_turns")
-    else
-        -- Placed elsewhere than in Taps and gestures, with only a subset of menu items.
-        self.menu_items.page_turns_non_touch = require("ui/elements/page_turns")
     end
+    -- While also related to page turns, this mostly concerns platform-specific features or physical keys, and applies *everywhere*
+    self.menu_items.platform_navigation_features = require("ui/elements/platform_navigation")
     -- insert DjVu render mode submenu just before the last entry (show advanced)
     -- this is a bit of a hack
     if self.ui.document.is_djvu then

frontend/ui/elements/filemanager_menu_order.lua

@@ -70,6 +70,8 @@ local order = {
         "back_in_reader",
         "backspace_as_back",
         "----------------------------",
+        "platform_navigation_features",
+        "----------------------------",
         "android_volume_keys",
         "android_haptic_feedback",
         "android_back_button",

frontend/ui/elements/page_turns.lua

@@ -1,4 +1,3 @@
-local Device = require("device")
 local Event = require("ui/event")
 local ReaderUI = require("apps/reader/readerui")
 local UIManager = require("ui/uimanager")
@@ -171,38 +170,4 @@ When enabled the UI direction for the Table of Contents, Book Map, and Page Brow
     }
 }
 
-if Device:canDoSwipeAnimation() then
-    table.insert(PageTurns.sub_item_table, {
-        text =_("Page turn animations"),
-        checked_func = function()
-            return G_reader_settings:isTrue("swipe_animations")
-        end,
-        callback = function()
-            UIManager:broadcastEvent(Event:new("TogglePageChangeAnimation"))
-        end,
-        separator = true,
-    })
-end
-
-if Device:hasKeys() then
-    table.insert(PageTurns.sub_item_table, {
-        text = _("Invert page turn buttons"),
-        checked_func = function()
-            return G_reader_settings:isTrue("input_invert_page_turn_keys")
-        end,
-        callback = function()
-            UIManager:broadcastEvent(Event:new("SwapPageTurnButtons"))
-        end,
-    })
-end
-
-if not Device:isTouchDevice() then
-    if Device:hasKeys() then
-        -- We just need the last item added above.
-        PageTurns = PageTurns.sub_item_table[#PageTurns.sub_item_table]
-    else
-        PageTurns = {} -- no menu item
-    end
-end
-
 return PageTurns

frontend/ui/elements/platform_navigation.lua

@@ -0,0 +1,41 @@
+local Device = require("device")
+local Event = require("ui/event")
+local UIManager = require("ui/uimanager")
+local _ = require("gettext")
+
+local PlatformNav = {
+    text = _("Page turn behavior"), -- Mainly so as to differentiate w/ "Page Turns" when in readermenu...
+    sub_item_table = {},
+}
+
+if Device:canDoSwipeAnimation() then
+    table.insert(PlatformNav.sub_item_table, {
+        text =_("Page turn animations"),
+        checked_func = function()
+            return G_reader_settings:isTrue("swipe_animations")
+        end,
+        callback = function()
+            UIManager:broadcastEvent(Event:new("TogglePageChangeAnimation"))
+        end,
+        separator = true,
+    })
+end
+
+if Device:hasKeys() then
+    table.insert(PlatformNav.sub_item_table, {
+        text = _("Invert page turn buttons"),
+        checked_func = function()
+            return G_reader_settings:isTrue("input_invert_page_turn_keys")
+        end,
+        callback = function()
+            UIManager:broadcastEvent(Event:new("SwapPageTurnButtons"))
+        end,
+    })
+end
+
+-- No menu item at all if it's empty
+if #PlatformNav.sub_item_table == 0 then
+    return {}
+end
+
+return PlatformNav

frontend/ui/elements/reader_menu_order.lua

@@ -114,7 +114,8 @@ local order = {
         "back_in_reader",
         "backspace_as_back",
         "----------------------------",
-        "page_turns_non_touch",
+        "platform_navigation_features",
+        "----------------------------",
         "android_volume_keys",
         "android_haptic_feedback",
         "android_back_button",