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/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