Home: the sign-in screen should keep app windows open instead of dropping them #497

When a Home session expires (12 hours) or an authentication error sends Home back to sign-in, Home clears every app context and blanks the desktop frame, so open apps lose their state. With Documents' Autosave off (the 0.8.0 default), unsaved edits are lost this way. A computer's lock screen keeps apps running underneath; Home's sign-in and lock should do the same: show sign-in over the desktop, refresh the frames' authority after the same person signs in, and only tear windows down (after asking) when a different person signs in.

Found while testing 0.8.0-alpha.27 (28 Oct). Planned for 0.8.1. Needs a security review of the lock semantics.