Fix that welcome mode was no longer shown at startup

Even though the tab was selected. Broke with
e440e344ca and
abcdadd40e .
We need to update the current index when tabs/modes are inserted before
it.

Change-Id: I26b5252d365c0108584534bcb0ee1aacc3eacce3
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/coreplugin/fancytabwidget.h

@@ -98,6 +98,8 @@ public:
         tab->text = label;
         tab->hasMenu = hasMenu;
         m_tabs.insert(index, tab);
+        if (m_currentIndex >= index)
+            ++m_currentIndex;
         updateGeometry();
     }
     void setEnabled(int index, bool enabled);