Add new color zenburn-bg+05 and use it to highlight current magit sections

I think the gap from zenburn-bg to zenburn-bg+1 is to big for "the next lighter background" to be appropriate to highlight something except for temporary selections.

But that's just what we have been doing for `magit-item-highlight` for quite some time now, and I would imagine that such a "just a bit lighter than the default" background color would be useful in other contexts too.