show total time in balancing target arenas

src/balancing/ArenaWidget.cpp

@@ -33,6 +33,9 @@ void ArenaWidget::buildLayout(const std::string& arenaName)
     m_titleLabel->setFont(titleFont);
     titleRow->addWidget(m_titleLabel);
 
+    m_durationLabel = new QLabel(this);
+    titleRow->addWidget(m_durationLabel);
+
     titleRow->addStretch();
 
     m_inspectButton = new QPushButton(tr("Inspect"), this);
@@ -159,6 +162,10 @@ void ArenaWidget::updateStatus(const ArenaStatus& status)
         content->setText(lines);
     }
 
+    m_durationLabel->setText(status.finished
+        ? tr("Duration: %1 s").arg(QString::number(status.durationSeconds, 'f', 1))
+        : QString());
+
     if (status.finished && !m_wasFinished)
     {
         m_wasFinished = true;