[Geary] Compose-cache

[johnywhy gmail com]

I couldn't find these in the tracker:

Request: During compose, auto-backup to a temporary cache would enable recovery of open editing sessions in case of crash or power-loss. This is different than writing over the currently-saved draft, which is undesirable. 

Bug? If i delete an open draft, i can't find it in Trash folder. Tested with gmail. Seems high-priority, since risk of accidental data-loss. ("high-priority" doesn't imply "time-sensitive", i understand geary is volunteer-built, and work is done if/when volunteers choose).