MenuSorter: review comments and various bug fixes

* fixed wrongful retention of submenus variable and added return to MenuSorter:findById
* fixed readerfooter_spec.lua error
* fixed review comments

frontend/apps/reader/modules/readerbookmark.lua

@@ -45,7 +45,7 @@ end
 
 function ReaderBookmark:addToMainMenu(tab_item_table)
     -- insert table to main reader menu
-    self.ui.menu.menu_items["bookmarks"] = {
+    self.ui.menu.menu_items.bookmarks = {
         text = self.bm_menu_title,
         callback = function()
             self:onShowBookmark()