add and test for DC_IMAP_MESSAGE_DELETED event

deltachat-ffi/deltachat.h

@@ -3904,6 +3904,16 @@ int64_t          dc_lot_get_timestamp     (const dc_lot_t* lot);
  */
 #define DC_EVENT_SMTP_MESSAGE_SENT        103
 
+/**
+ * Emitted when a message was successfully marked as deleted on the SMTP server.
+ *
+ * @param data1 0
+ * @param data2 (const char*) Info string in english language.
+ *     Must not be free()'d or modified and is valid only until the callback returns.
+ * @return 0
+ */
+#define DC_EVENT_IMAP_MESSAGE_DELETED   104
+
 
 /**
  * The library-user should write a warning string to the log.