[enh] google engine: supports "default language"

Same behaviour behaviour than Whoogle [1].  Only the google engine with the
"Default language" choice "(all)"" is changed by this patch.

When searching for a locate place, the result are in the expect language,
without missing results [2]:

  > When a language is not specified, the language interpretation is left up to
  > Google to decide how the search results should be delivered.

The query parameters are copied from Whoogle.  With the ``all`` language:

- add parameter ``source=lnt``
- don't use parameter ``lr``
- don't add a ``Accept-Language`` HTTP header.

The new signature of function ``get_lang_info()`` is:

    lang_info = get_lang_info(params, lang_list, custom_aliases, supported_any_language)

Argument ``supported_any_language`` is True for google.py and False for the other
google engines.  With this patch the function now returns:

- query parameters: ``lang_info['params']``
- HTTP headers: ``lang_info['headers']``
- and as before this patch:
  - ``lang_info['subdomain']``
  - ``lang_info['country']``
  - ``lang_info['language']``

[1] https://github.com/benbusby/whoogle-search
[2] https://github.com/benbusby/whoogle-search/releases/tag/v0.5.4

searx/engines/google_images.py

@@ -101,14 +101,13 @@ def request(query, params):
 
     lang_info = get_lang_info(
         # pylint: disable=undefined-variable
-        params, supported_languages, language_aliases
+        params, supported_languages, language_aliases, False
     )
 
     query_url = 'https://' + lang_info['subdomain'] + '/search' + "?" + urlencode({
         'q': query,
         'tbm': "isch",
-        'hl': lang_info['hl'],
-        'lr': lang_info['lr'],
+        **lang_info['params'],
         'ie': "utf8",
         'oe': "utf8",
         'num': 30,
@@ -122,8 +121,8 @@ def request(query, params):
     logger.debug("query_url --> %s", query_url)
     params['url'] = query_url
 
-    logger.debug("HTTP header Accept-Language --> %s", lang_info['Accept-Language'])
-    params['headers']['Accept-Language'] = lang_info['Accept-Language']
+    logger.debug("HTTP header Accept-Language --> %s", lang_info.get('Accept-Language'))
+    params['headers'].update(lang_info['headers'])
     params['headers']['Accept'] = (
         'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8'
     )