Convert Application to a functional component

This makes properties and state type-safe, i.e. work better for
TypeScript projects.

Also close the file watch properly after the component unmounts. That
should never happen in practice for *this case*, but this is
example/model code, so let's be correct.

src/app.jsx

@@ -17,7 +17,7 @@
  * along with Cockpit; If not, see <http://www.gnu.org/licenses/>.
  */
 
-import React from 'react';
+import React, { useEffect, useState } from 'react';
 import { Alert } from "@patternfly/react-core/dist/esm/components/Alert/index.js";
 import { Card, CardBody, CardTitle } from "@patternfly/react-core/dist/esm/components/Card/index.js";
 
@@ -25,27 +25,24 @@ import cockpit from 'cockpit';
 
 const _ = cockpit.gettext;
 
-export class Application extends React.Component {
-    constructor() {
-        super();
-        this.state = { hostname: _("Unknown") };
+export const Application = () => {
+    const [hostname, setHostname] = useState(_("Unknown"));
 
-        cockpit.file('/etc/hostname').watch(content => {
-            this.setState({ hostname: content.trim() });
+    useEffect(() => {
+        const hostname = cockpit.file('/etc/hostname');
+        hostname.watch(content => setHostname(content.trim()));
+        return hostname.close;
     });
-    }
 
-    render() {
     return (
         <Card>
             <CardTitle>Starter Kit</CardTitle>
             <CardBody>
                 <Alert
                     variant="info"
-                        title={ cockpit.format(_("Running on $0"), this.state.hostname) }
+                    title={ cockpit.format(_("Running on $0"), hostname) }
                 />
             </CardBody>
         </Card>
     );
-    }
-}
+};