Welcomepage: Is should not be possible to uncheck tabs

Unchecking a tab has no clear semantic.

Change-Id: I9bb3d659a76655846c79cabbe23717119ed0cf08
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

share/qtcreator/welcomescreen/widgets/Tabs.qml

@@ -51,6 +51,7 @@ Column {
 
             onClicked: {
                 customTab.currentIndex = index
+                checked = true
             }
         }
     }