Support separate menu selection per active theme

I have been using this module and it is fantastic. But on a current client site I'm working on, we have the use case where a node is linked in two separate menus, once in the 'Primary Links' menu and also in the 'Secondary Links' menu. However, only the link in the 'Secondary Links' menu also has all the children for this link populated as well, therefore that is the link we would want to use to set the breadcrumbs for this page and all of its children. Now, obviously, there's no way to set different breadcrumbs per individual page with this module or otherwise, as that would be kludgy and undesirable.

Fortunately, it is also the case that visiting the page for this node triggers the activation of a different theme. Thus I had the idea that to accommodate this use case, the module could allow you to select a different menu per active theme to use when settings the breadcrumbs. I do realize my use case is probably fairly uncommon, but I also don't think this change is very dramatic in terms of usability or the codebase. That is of course for you to decide though.

I also have already implemented this change in my own local copy of the module and will attach my patch to this issue for your review.