Support `XDG_STATE_HOME`

[horriblename]

from the specification:

The $XDG_STATE_HOME contains state data that should persist between (application) restarts, but that is not important or portable enough to the user that it should be stored in $XDG_DATA_HOME. It may contain:

• actions history (logs, history, recently used files, …)

• current state of the application that can be reused on a restart (view, layout, open files, undo history, …)