html images: copy alt to title if no title

[t3chguy]

so there is more chance there is a textual description available on-hover

[dbkr]

I think this is still relevant, although I'd also sort of think it would be up to the browser to use the alt if a title isn't set? If it doesn't, it's probably for a reason and I'm not sure it makes sense for us to mess with it.