Allow non-string headers in TabbedPanel

Currently TabbedPanel accepts only string headers. However, since it simply renders the header, accepting other React elements should be possible, and it would be useful to display SVG icons.

As an example, passing a header like
```jsx
(
  <View shrink={false} vAlignContent="bottom">
    <MySvgIcon/>
    {label}
  </View>
) as any,
```
(bypassing type checking) seems to work OK.