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

Main changes:
- The plugin no longer uses the "Remote database" profile key, since
  this was previously used to refer to the sole calendar to sync for
  the profile.
- The list of calendars for to be synced for a profile is now read
  from the account's service settings
- Each notebook (i.e. calendar) added for the account is now 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.