Remove platform option and use HUD option instead

This patch does two related things:

* Removes the "platform" option. This was replaced with 4 quality values, which correspond with old platform+quality combinations. The code was modified so that the same decision points are still applied, but are now just based on the quality option.
* Adds a separate HUD option. Rather than disable the HUD for mobile automatically, it's now possible to have the HUD alongside mobile-quality graphics.

NB the HUD does still degrade performance significantly, but at least you can switch it on for mobile (while retaining all the other graphical optimisations for mobile). I might look at improving the HUD next.

Relates to #21