Implement sort_title getter, setter

src/library/entry.cr

@@ -8,6 +8,9 @@ class Entry
     size : String, pages : Int32, id : String, encoded_path : String,
     encoded_title : String, mtime : Time, err_msg : String?
 
+  @[YAML::Field(ignore: true)]
+  @sort_title : String?
+
   def initialize(@zip_path, @book)
     storage = Storage.default
     @encoded_path = URI.encode @zip_path
@@ -66,6 +69,27 @@ class Entry
     end
   end
 
+  def sort_title
+    sort_title_cached = @sort_title
+    return sort_title_cached if sort_title_cached
+    sort_title = Storage.default.get_entry_sort_title id
+    if sort_title
+      @sort_title = sort_title
+      return sort_title
+    end
+    @sort_title = @title
+    @title
+  end
+
+  def sort_title=(sort_title : String | Nil)
+    Storage.default.set_entry_sort_title id, sort_title
+    @sort_title = sort_title
+  end
+
+  def sort_title_db
+    Storage.default.get_entry_sort_title id
+  end
+
   def display_name
     @book.display_name @title
   end