Remove deaddrop chat

Contact request chats are not merged into a single virtual "deaddrop"
chat anymore. Instead, they are shown in the chatlist the same way as
other chats, but sending of messages to them is not allowed and MDNs
are not sent automatically until the chat is "accepted" by the user.

New API:
- dc_chat_is_contact_request(): returns true if chat is a contact
request.  In this case option to accept and block the chat via
dc_accept_chat() and dc_block_chat() should be shown in the UI.
- dc_accept_chat(): accept contact request and unblock the chat
- dc_block_chat(): decline contact request and block the chat

Removed API:
- dc_create_chat_by_msg_id(): deprecated 2021-02-07 in favor of
  dc_decide_on_contact_request()
- dc_marknoticed_contact(): deprecated 2021-02-07 in favor of
  dc_decide_on_contact_request()
- dc_decide_on_contact_request(): this call requires a message ID from
  deaddrop chat as input. As deaddrop chat is removed, this call can't
  be used anymore.
- dc_msg_get_real_chat_id(): use dc_msg_get_chat_id() instead, the
  only difference between these calls was in handling of deaddrop chat
- removed DC_CHAT_ID_DEADDROP and DC_STR_DEADDROP constants

python/src/deltachat/hookspec.py

@@ -43,7 +43,7 @@ class PerAccount:
 
     @account_hookspec
     def ac_incoming_message(self, message):
-        """ Called on any incoming message (to deaddrop or chat). """
+        """ Called on any incoming message (both existing chats and contact requests). """
 
     @account_hookspec
     def ac_outgoing_message(self, message):