Fix TabView not to insert duplicate tabs

There are three ways to add tabs:
- declare 'static' Tab { } child elements
- call addTab() / insertTab()
- Component::createObject(tabView)

=> Do not rely on the calling order of Component.onCompleted()
and make sure that the same tabs are not accidentally re-inserted.

Task-number: QTBUG-30873
Change-Id: Ib30cfb676debbb302c5e9f7d757f66aab6fcc684
Reviewed-by: Caroline Chao <caroline.chao@digia.com>