[UX] Adds "Enable back history" menu item (#4443)

For a hidden setting already present, and used in
/apps/reader/modules/readerback.lua
Only shown, as the whole Navigation> submenu, on
devices with keys.

frontend/ui/elements/common_settings_menu_table.lua

@@ -190,6 +190,15 @@ if Device:hasKeys() then
                     },
                 },
             },
+            {
+                text = _("Enable back history"),
+                checked_func = function()
+                    return G_reader_settings:nilOrTrue("enable_back_history")
+                end,
+                callback = function()
+                    G_reader_settings:flipNilOrTrue("enable_back_history")
+                end,
+            },
         }
     }
 end