Change Instantiator to Repeater in NavigationTabBar

The Instantiator was causing issues, since NavigationTabButtons that
were supposed to be destroyed weren't really destroyed and still showed
up as buttons in the tabGroup.  This was causing infinite loops in
several apps under certain circumstances (plasma dialer, koko, discover,
kclock, kasts, tokodon).

See plasma-mobile/plasma-dialer#48 and
utilities/kclock#72

CCBUG: 467860