[feat] Adds Document> Auto-save book metadata sub-menu (#5687)

Replaces DAUTO_SAVE_PAGING_COUNT in defaults.lua with
a new option available in menu, that allows setting
an interval in minutes instead of a page count.

frontend/ui/elements/common_settings_menu_table.lua

@@ -345,9 +345,76 @@ if Device:hasKeys() then
     }
 end
 
+-- Auto-save settings: default value, info text and warning, and menu items
+if G_reader_settings:readSetting("auto_save_settings_interval_minutes") == nil then
+    -- Default to auto save every 15 mn
+    G_reader_settings:saveSetting("auto_save_settings_interval_minutes", 15)
+end
+
+local auto_save_help_text = _([[
+This sets how often to rewrite to disk global settings and book metadata, including your current position and any highlights and bookmarks made, when you're reading a document.
+
+The normal behavior is to save those only when the document is closed, or your device suspended, or when exiting KOReader.
+
+Setting it to some interval may help prevent losing new settings/sidecar data after a software crash, but will cause more I/O writes the lower the interval is, and may slowly wear out your storage media in the long run.]])
+
+-- Some devices with FAT32 storage may not like having settings rewritten too often,
+-- so let that be known. See https://github.com/koreader/koreader/pull/3625
+local warn_about_auto_save = Device:isKobo() or Device:isKindle() or Device:isCervantes() or Device:isPocketBook() or Device:isSonyPRSTUX()
+if warn_about_auto_save then
+    local auto_save_help_warning = _([[Please be warned that on this device, setting a low interval may exacerbate the potential for filesystem corruption and complete data loss after a hardware crash.]])
+    auto_save_help_text = auto_save_help_text .. "\n\n" .. auto_save_help_warning
+end
+
+local function genAutoSaveMenuItem(value)
+    local setting_name = "auto_save_settings_interval_minutes"
+    local text
+    if not value then
+        text = _("Only on close, suspend and exit")
+    else
+        text = T(_("Every %1 mn"), value)
+    end
+    return {
+        text = text,
+        help_text = auto_save_help_text,
+        checked_func = function()
+            return G_reader_settings:readSetting(setting_name) == value
+        end,
+        callback = function()
+            G_reader_settings:saveSetting(setting_name, value)
+        end,
+    }
+end
+
 common_settings.document = {
     text = _("Document"),
     sub_item_table = {
+        {
+            text_func = function()
+                local interval = G_reader_settings:readSetting("auto_save_settings_interval_minutes")
+                local s_interval
+                if interval == false then
+                    s_interval = "only on close"
+                else
+                    s_interval = T(_("every %1 mn"), interval)
+                end
+                return T(_("Auto-save book metadata: %1"), s_interval)
+            end,
+            help_text = auto_save_help_text,
+            sub_item_table = {
+                genAutoSaveMenuItem(false),
+                genAutoSaveMenuItem(5),
+                genAutoSaveMenuItem(15),
+                genAutoSaveMenuItem(60),
+                warn_about_auto_save and {
+                    text = _("Important info about this auto-save option"),
+                    keep_menu_open = true,
+                    callback = function()
+                        UIManager:show(InfoMessage:new{ text = auto_save_help_text, })
+                    end,
+                } or nil,
+            },
+        },
         {
             text = _("Save document (write highlights into PDF)"),
             sub_item_table = {