add a device message when backup transfer ends

that way, UI can just close the transfer dialog,
so that, at the end, both devices end in the chatlist.

we can also use this for troubleshooting -
if the device message is not present,
transfer did not succeed completely.

(a separate device message may be nice in that case -
but that is another effort,
same for making the device message reappear after deletion
or after some time)

deltachat-ffi/deltachat.h

@@ -7069,6 +7069,11 @@ void dc_event_unref(dc_event_t* event);
 /// `%1$s` will be replaced by name and address of the account.
 #define DC_STR_BACKUP_TRANSFER_QR 162
 
+/// "Account transferred to your second device."
+///
+/// Used as a device message after a successful backup transfer.
+#define DC_STR_BACKUP_TRANSFER_MSG_BODY 163
+
 /**
  * @}
  */