Add "Auto-detected" as a language.

When the user choose "Auto-detected", the choice remains on the following queries.
The detected language is displayed.

For example "Auto-detected (en)":
* the next query language is going to be auto detected
* for the current query, the detected language is English.

This replace the autodetect_search_language plugin.

searx/search/models.py

@@ -109,3 +109,16 @@ class SearchQuery:
                 self.external_bang,
             )
         )
+
+    def __copy__(self):
+        return SearchQuery(
+            self.query,
+            self.engineref_list,
+            self.lang,
+            self.safesearch,
+            self.pageno,
+            self.time_range,
+            self.timeout_limit,
+            self.external_bang,
+            self.engine_data,
+        )