Add desmume version string to the about box

This change will add the the DeSmuMe version string to the about box, in addition to the nds4ios version string.

I've only done it in the English version of the storyboard at the moment. I can either 
a) add it into every storyboard file, which is a pain, as every UI change requires multiple storyboard updates
 or
b) pull all the localisation out into .strings files, which refer to the labels in just one base storyboard file, which is much more manageable for future localisation or storyboard changes in the future, but would require dropping iOS 5 support, which I don't want to do without permission!