[vdr] Different handling of back / menu keys

The VDR menu key is somewhat under-utilised, it only toggles the menu on and off, I think it would be an improvement if it was combined with the back key.

Instead of using the menu key to toggle the menu on and off, it could trigger the osd menu initially, initially, but would then subsequently work as a back key. So if you're deep in a menu, and you press the menu key, it would take you back one level, just as the back key does.

If you need to go fully out, you long-press the menu key.

This would mostly eliminate the need for a dedicated back key, and would simplify the basic menu navigation and make it more in line with other menu navigation systems (eg the ipod comes to mind).