add test and python API for verified group handling/chatting

add msg.is_encrypted() API and check for it from some tests

python/src/deltachat/chatting.py

@@ -109,6 +109,13 @@ class Chat(object):
         """
         return not lib.dc_chat_is_unpromoted(self._dc_chat)
 
+    def is_verified(self):
+        """ return True if this chat is a verified group.
+
+        :returns: True if chat is verified, False otherwise.
+        """
+        return lib.dc_chat_is_verified(self._dc_chat)
+
     def get_name(self):
         """ return name of this chat.