Add a TitleBar::isFocused property so we can use with qss

This has nothing to do with QWidget::hasFocus, but regarding our
focus scope support

src/private/TitleBar_p.h

@@ -43,6 +43,7 @@ class DOCKS_EXPORT TitleBar : public QWidgetAdapter, public Draggable
     Q_PROPERTY(bool closeButtonEnabled READ closeButtonEnabled WRITE setCloseButtonEnabled NOTIFY closeButtonEnabledChanged)
     Q_PROPERTY(bool floatButtonVisible READ floatButtonVisible WRITE setFloatButtonVisible NOTIFY floatButtonVisibleChanged)
     Q_PROPERTY(QString floatButtonToolTip READ floatButtonToolTip NOTIFY floatButtonToolTipChanged)
+    Q_PROPERTY(bool isFocused READ isFocused NOTIFY isFocusedChanged)
 public:
     typedef QVector<TitleBar *> List;