[TIMOB-3376] left- and rightNavButton fade in

When adding a left or rightNavButton to a window, the button fades into view. The expected behavior would be for the button to become immediately visible when the window/tabGroup opens:

var tabGroup = Titanium.UI.createTabGroup();
var win1 = Titanium.UI.createWindow({  
    title:'Tab 1',
    backgroundColor:'#fff'
});
var tab1 = Titanium.UI.createTab({  
    icon:'KS_nav_views.png',
    title:'Tab 1',
    window:win1
});

win1.setRightNavButton(Ti.UI.createButton({
    title:'foobar'
}));

var win2 = Titanium.UI.createWindow({  
    title:'Tab 2',
    backgroundColor:'#fff'
});
var tab2 = Titanium.UI.createTab({  
    icon:'KS_nav_ui.png',
    title:'Tab 2',
    window:win2
});

tabGroup.addTab(tab1);  
tabGroup.addTab(tab2);
tabGroup.open();