Add toggle own nodes permissions

If users of a certain role are granted permission to toggle posts, they are able to toggle the posts of other users with the same role. It would be useful to have a further degree of granularity like "promote own posts", so the user would be restricted to being able to toggle content they own.

I realize this tool is more for editors and moderators than individual users, but the convenience it gives users is very nice, and this would allow them more flexibilty for quick status changes.