Make frontpage callback optional

I'm working on a site that uses a custom frontpage callback method. With the weight module installed, this is being ignored and causes the 'No front page content has been created' message to appear on the homepage.

Attached is a patch to make the Weight module's frontpage callback optional, adding an additional 'add to frontpage' field on the weight configuration form. I'd rather this be a global option, rather than per content type, but wasn't sure how that would be done.