add editable "summary" to dc_msg_get_webxdc_info()

the summary can be modified by the apps using
`sendUpdate({summary: "foo", payload: ...})`

the summary is updated when there is no newer update
and chat will be informed by the change as usual by
`DC_EVENT_MSGS_CHANGED`.

deltachat-ffi/deltachat.h

@@ -3701,6 +3701,9 @@ char*             dc_msg_get_webxdc_blob      (const dc_msg_t* msg, const char*
  *   To get the file, use dc_msg_get_webxdc_blob().
  *   App icons should should be square,
  *   the implementations will add round corners etc. as needed.
+ * - summary: short string describing the state of the app,
+ *   sth. as "2 votes", "Highscore: 123",
+ *   can be changed by the apps and defaults to an empty string.
  *
  * @memberof dc_msg_t
  * @param msg The webxdc instance.