State Notes: Allow capturing state notes in a drawer, LOGBOOK

State Notes: Allow capturing state notes in a drawer, LOGBOOK

Wanrong Lin writes:

     2. Can we put all of those state change history logging messages
        into a drawer like "STATE-HIST" (when the drawer is defined in
        org-drawers), so it is less intrusive in the file?

Yes we can.  This commit provides this functionality.  It introduces a
new drawer, LOGBOOK.  When the variable
`org-log-state-notes-into-drawer' is set, state change time stamps and
notes will go into this drawer, or another one specified by that
variable.