[caldav] Add support for syncing multiple calendars for a single profile/account.

Main changes:
- The list of calendars for to be synced for a profile is read from the
  account's service settings
- Each notebook (i.e. calendar) added for the account is identified by
  calling Notebook::setAccount() with the string
  "<account_id>-<remote_calendar_path>". For example,
  "35-/dav/fluffybunny/Calendar/Work_Calendar/'
- The storage loading and saving is now all done by CalDavClient to
  ensure it is is only done once per sync session

Also added some brief docs for syncing an account/profile with this
plugin.