fix recording of Equake buffer history
nil, then history of Equake frame does not seem to work properly. Instead of actual Equake buffers being restored, another buffer appears in the Equake frame. I think this is because frame destruction in some contexts leaves behind records of dead buffers which get improperly indexed. A potential fix seems to be to check whether a buffer is actually an Equake buffer before storing a record for it. The drawback is that if a user had a non-Equake buffer open in Equake when they "hide" the Equake frame, it will not re-appear automatically when Equake is re-invoked. Still, defaulting to restoring only Equake buffers seems like a better outcome than an arbitrary Emacs buffer appearing when Equake is re-opened.