Repurposed field 'oneshot' in units table into field 'unread'.
New feature: read/unread unit tracking.

When the server loads a unit, the "unread" field in that unit's description is
checked. If unread == true, the description is changed to unread = false
and the saved to the DB.

When the client receives a successful "load_unit" response, it checks the
unit description's 'unread' field. If unread == true, the client sends a
get_unit IPC to the server to request an updated unit description and
updates the library model and view with the new unit description.