Post-Basic-Features: Allow temporary caching of encounters / loading existing encounters

Putting this here so I can hold myself accountable. It would be interesting to expand the parser so it is able to open existing encounters, so that it is possible to edit them with this app once they are created. It would also be nice to be able to save encounters, so that one
can pause editing and pick it up again before the encounter is finished completely.

Start by locating the parser and the current encounter-creation flow. Read how encounters are represented and held while editing, then determine the expected behavior for opening an existing encounter and resuming a partially completed one. Done should include both loading existing encounters and temporarily saving and restoring unfinished encounters.

Written by the indexing model from the issue text.