AltStatusBar/Footer: add the read timer value (#12002)

Closes #11950

frontend/apps/reader/modules/readerfooter.lua

@@ -341,10 +341,10 @@ local footerTextGeneratorMap = {
             prefix .. " ", left)
     end,
     mem_usage = function(footer)
-        local symbol_type = footer.settings.item_prefix
-        local prefix = symbol_prefix[symbol_type].mem_usage
         local statm = io.open("/proc/self/statm", "r")
         if statm then
+            local symbol_type = footer.settings.item_prefix
+            local prefix = symbol_prefix[symbol_type].mem_usage
             local dummy, rss = statm:read("*number", "*number")
             statm:close()
             -- we got the nb of 4Kb-pages used, that we convert to MiB
@@ -516,6 +516,8 @@ ReaderFooter.default_settings = {
 function ReaderFooter:init()
     self.settings = G_reader_settings:readSetting("footer", self.default_settings)
 
+    self.additional_footer_content = {} -- place, where additional header content can be inserted.
+
     -- Remove items not supported by the current device
     if not Device:hasFastWifiStatusQuery() then
         MODE.wifi_status = nil
@@ -1981,6 +1983,19 @@ function ReaderFooter:genAlignmentMenuItems(value)
     }
 end
 
+function ReaderFooter:addAdditionalFooterContent(content_func)
+    table.insert(self.additional_footer_content, content_func)
+end
+
+function ReaderFooter:removeAdditionalFooterContent(content_func)
+    for i, v in ipairs(self.additional_footer_content) do
+        if v == content_func then
+            table.remove(self.additional_footer_content, i)
+            return true
+        end
+    end
+end
+
 -- this method will be updated at runtime based on user setting
 function ReaderFooter:genFooterText() end
 
@@ -2153,6 +2168,13 @@ function ReaderFooter:_updateFooterText(force_repaint, full_repaint)
         return
     end
     local text = self:genFooterText()
+    for dummy, v in ipairs(self.additional_footer_content) do
+        local value = v()
+        if value and value ~= "" then
+            text = value .. " " .. self:get_separator_symbol() .. " " .. text
+        end
+    end
+
     if not text then text = "" end
     self.footer_text:setText(text)
     if self.settings.disable_progress_bar then