add squeekboard toggle button

shell.qml

@@ -63,6 +63,27 @@ ShellRoot {
         }
     }
 
+    // Not a menu: this one drives squeekboard, so it is worth binding to a
+    // hardware key or a tablet-mode switch as well as clicking.
+    IpcHandler {
+        target: "osk"
+
+        function toggle(): void {
+            Osk.toggle();
+        }
+
+        // Named to match the menu targets above: `qs ipc call` swallows a
+        // function called `show` — it collides with the `ipc show` subcommand
+        // — and a lone `hide` next to `open` would read oddly anyway.
+        function open(): void {
+            Osk.show();
+        }
+
+        function close(): void {
+            Osk.hide();
+        }
+    }
+
     IpcHandler {
         target: "bluetooth"