Delete all buffers that are not part of the current project

`BobOpen` makes opening buffers very easy. Closing buffers is not so easy,
currently. When switching to a new project, open buffers from the previous
project are still open and might make the user edit the wrong file in the worst
case. For that it would be handy to have a command `BobCloseUnrelated` or
similar, to close (delete) all buffers that are not part of the current project.