Tweak SleepCover handling a bit on Kobo

Specifically, how we ignore it.
No longer do anything specific and potentially racy, and let the
unexpected wakeup handler take care of putting us back to sleep.

Also add an option to only ignore wakeup events from the SleepCover.

Re #5098
Re #5087

frontend/ui/elements/reader_menu_order.lua

@@ -60,7 +60,8 @@ local order = {
         "time",
         "battery",
         "autosuspend",
-        "sleepcover",
+        "ignore_sleepcover",
+        "ignore_open_sleepcover",
         "mass_storage_settings",
     },
     navigation = {