keyboard: Add visible property

So its visibility can be queried externally.

js/ui/keyboard.js

@@ -301,6 +301,10 @@ var Keyboard = new Lang.Class({
         this._redraw();
     },
 
+    get visible() {
+        return this._keyboardVisible;
+    },
+
     _setCaretTrackerEnabled: function (enabled) {
         if (this._caretTrackingEnabled == enabled)
             return;