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`.

draft/webxdc-dev-reference.md

@@ -33,6 +33,9 @@ To get a shared state, the peers use `sendUpdate()` to send updates to each othe
        eg. "Alice voted" or "Bob scored 123 in MyGame";
        usually only one line of text is shown,
        use this option sparingly to not spam the chat.
+    - `update.summary`: optional, short text, shown beside app icon;
+       it is recommended to use some aggregated value,  eg. "8 votes", "Highscore: 123"
+
 - `descr`: short, human-readable description what this update is about.
   this is shown eg. as a fallback text in an email program.