NavigationTabBar: Avoid loop on actions in tab group
The actions in Kirigami.NavigationTabBar are bounded by an exclusive button group, which on start-up competes with the binding on the `checked` property within those actions. This results in an endless loop of alternating states, and eventually a crash when the QV4 heap saturates. Setting this group as non-exclusive fixes the issue. BUG: 448460 FIXED-IN: 5.24