Make the blink effect work with QToolButton

It's not perfect but we're not after perfection. It works.

See longturn#940.

client/gui-qt/top_bar.cpp

@@ -172,22 +172,18 @@ void top_bar_widget::paintEvent(QPaintEvent *event)
   const auto highlight_rect =
       QRectF(0.5, 0, width() - 1. / devicePixelRatio(),
              height() - 1. / devicePixelRatio());
+  p.end();
 
-  if (current) {
-    p.setPen(QPen(palette().color(QPalette::Highlight), 0));
-    p.drawRect(highlight_rect);
-  }
+  QToolButton::paintEvent(event);
 
   if (blink) {
-    p.setCompositionMode(QPainter::CompositionMode_ColorDodge);
-    p.setPen(QPen(Qt::black, 0));
+    p.begin(this);
+    p.setPen(Qt::NoPen);
+    p.setCompositionMode(QPainter::CompositionMode_SoftLight);
     p.setBrush(palette().color(QPalette::HighlightedText));
     p.drawRect(highlight_rect);
+    p.end();
   }
-
-  p.end();
-
-  QToolButton::paintEvent(event);
 }
 
 /**