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`.

src/param.rs

@@ -171,6 +171,12 @@ pub enum Param {
 
     /// For Chats: timestamp of protection settings update.
     ProtectionSettingsTimestamp = b'L',
+
+    /// For Webxdc Message Instances: Current summary
+    WebxdcSummary = b'N',
+
+    /// For Webxdc Message Instances: timestamp of summary update.
+    WebxdcSummaryTimestamp = b'Q',
 }
 
 /// An object for handling key=value parameter lists.