Screensaver option on Kindle devices (#2734)

* Screensaver on Kindle devices

frontend/apps/reader/modules/readermenu.lua

@@ -145,8 +145,27 @@ function ReaderMenu:setUpdateItemTable()
                 }
             }
         }
+    elseif Device:isKindle() then
+        self.menu_items.screensaver = {
+            text = _("Screensaver"),
+            sub_item_table = {
+                {
+                    text = _("Use book's cover as screensaver"),
+                    checked_func = Screensaver.isUsingBookCover,
+                    callback = function()
+                        if Screensaver:isUsingBookCover() then
+                            G_reader_settings:saveSetting(
+                                "use_lastfile_as_screensaver", false)
+                        else
+                            G_reader_settings:delSetting(
+                                "use_lastfile_as_screensaver")
+                        end
+                        G_reader_settings:flush()
+                    end
+                }
+            }
+        }
     end
-
     -- main menu tab
     -- insert common info
     for id, common_setting in pairs(require("ui/elements/common_info_menu_table")) do