Adds a headerItem property in case a header is desired for the menu

[vittoriom]

The headerItem is just a REMenuItem so it's very easy to create one. Moreover, it's just a property on the REMenu instance and it's back-compatible with the existing codebase since when no header is specified, the normal behavior is preserved.

I removed the combinedHeight computed property in favour of a manually set one (this could open the road to different heights for each menuItem - maybe through a dataSource protocol)