Consider saving/restoring windows' recent-buffers (e.g. tab-line)

[alphapapa]

See #116

[jdtsmith]

A related idea: keep a long-running list of buffers(/files) visited in any window of a given activity, and add that frequency-sorted list of "not visible but potentially interesting" buffers to activities-switch-buffer. I'm also not sure why the latter is only supported with tabs and not frames; ideally this would support both UI scenarios. The idea is the longer you use an activity, the better that "recently visited but not currently active" buffer list will be.