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/flightplancomponent.cpp

@@ -65,6 +65,7 @@ namespace BlackGui
 
             // fix style
             this->tabBar()->setExpanding(false);
+            this->tabBar()->setUsesScrollButtons(true);
 
             // validators
             CUpperCaseValidator *ucv = new CUpperCaseValidator(this);