audqt: Hide dotted rectangle around volume slider when focussed

See https://stackoverflow.com/q/62258966 for a screenshot.

This still occurs with Qt 6 and various themes on Linux.
Disabling the focus with `slider.setFocusPolicy(Qt.NoFocus)`
is not an option since we want to still handle wheel events.

src/libaudqt/volumebutton.cc

@@ -57,6 +57,18 @@ class VolumeSliderStyle : public QProxyStyle
 
         return styleHint;
     }
+
+    void drawPrimitive(PrimitiveElement element,
+                       const QStyleOption * option,
+                       QPainter * painter,
+                       const QWidget * widget) const override
+    {
+        /* hide dotted rectangle when slider is focussed */
+        if (element == QStyle::PE_FrameFocusRect)
+            return;
+
+        QProxyStyle::drawPrimitive(element, option, painter, widget);
+    }
 };
 
 class VolumeButton : public QToolButton