Add binding for win.dual-odd-left

Hi there,

I'd like to merge a new key-binding to allow the toggling of win.dual-odd-left (i.e. Odd pages left); I often find myself needing to display two pages in presentations, and the inability to switch it with a binding is a little annoying.

I don't know the codebase, so I've merely looked for a binding-related line for win.dual-page and adapted it. I haven't tested it out for lack of time, but I think the desire at least will be evident from the PR.

Thank you for your work on Evince!