We're using weakrefs here because I'm paranoid about the lifecycle
issues with GTK+ + Python refcounting fun.

Starting a group now means dereferencing and re-referencing these
buffers, which isn't great, but is probably the least-bad option. The
other choice here would be to give groups a reference to their parent
UndoSequence, and have the buffers lookup follow that, which seems...
more complicated and confusing.