Elevated and unelevated terminal windows prune eachothers' session states

[AndisGrossteins]

Description of the new feature

Currently, the session state buffers are saved for whichever window was closed last. If there is one unelevated and one elevated window, when both are closed (to apply Terminal updates or reboot system), buffers are saved for the instance that exited last.

This leads to loss of buffer history for one of the windows that were open during last session – usually the elevated window.

Proposed technical implementation details

The buffers could be kept for both as it's done for session states with state.json and elevated-state.json. The buffers could be written to a separate subdirectory under LocalState directory or using a filename prefix – elevated-{GUID}.txt.

[DHowett]

Whoops. When Terminal exits, it prunes any leftover buffers from previous runs that weren't present in its settings file. We're clearly too aggressive about that, and you're right in that it's because we can't tell apart which ones were for which session.