Recheck message actions on decrypt

This changes the message action bar to force an update after an event decrypts,
since it's quite likely that changes the set of available actions.

Fixes https://github.com/vector-im/riot-web/issues/9991

src/components/views/messages/MessageActionBar.js

@@ -36,6 +36,20 @@ export default class MessageActionBar extends React.PureComponent {
         onFocusChange: PropTypes.func,
     };
 
+    componentDidMount() {
+        this.props.mxEvent.on("Event.decrypted", this.onDecrypted);
+    }
+
+    componentWillUnmount() {
+        this.props.mxEvent.removeListener("Event.decrypted", this.onDecrypted);
+    }
+
+    onDecrypted = () => {
+        // When an event decrypts, it is likely to change the set of available
+        // actions, so we force an update to check again.
+        this.forceUpdate();
+    }
+
     onFocusChange = (focused) => {
         if (!this.props.onFocusChange) {
             return;