Explicitly use QTabBar scroll buttons

Even though QTabBar's property usesScrollButtons is set to true in Qt
Designer, it is not set explicitly in generated C++ code. The default
value is style dependent, which caused the scroll buttons to be visibile
everywhere except Mac OS/X.

refs #683

src/blackgui/components/settingscomponent.cpp

@@ -44,6 +44,7 @@ namespace BlackGui
         {
             ui->setupUi(this);
             this->tabBar()->setExpanding(false);
+            this->tabBar()->setUsesScrollButtons(true);
 
             // Opacity, intervals
             this->connect(this->ui->hs_SettingsGuiOpacity, &QSlider::valueChanged, this, &CSettingsComponent::changedWindowsOpacity);