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.

tests/unit/test_search.py

@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 
+from copy import copy
+
 import searx.search
 from searx.search import SearchQuery, EngineRef
 from searx import settings
@@ -34,6 +36,11 @@ class SearchQueryTestCase(SearxTestCase):
         self.assertEqual(s, s)
         self.assertNotEqual(s, t)
 
+    def test_copy(self):
+        s = SearchQuery('test', [EngineRef('bing', 'general')], 'all', 0, 1, None, None, None)
+        t = copy(s)
+        self.assertEqual(s, t)
+
 
 class SearchTestCase(SearxTestCase):
     @classmethod