oc-calendar: Retrieve properly event description created on SOGo

As it is stored as plain text without triming string.

The correct comparison here is NSNotFound as stated in Apple documentation.
oc-rtf: Use what RTF Spec 1.9.1 for control words

This reverts 7581c0a.
oc-rtf: Fix current position to match where _bytes pointer is

This avoids to have extra unknown bytes at the end of the RTF document
as _len would have been reached but _current_pos had incorrect less value.
oc-rtf: Be more resilient to spaces while parsing font tables

This avoids to crash on this example:

  \f0\fibi \fcharset0
   -------^

Or:

  \f0 \fibi\fcharset0
   --^

Take into account that I found lots of RTF documents with this format:

  \f0\fbidi \froman\fcharset0
  \f0\froman\fprq2 \fcharset0

Which are not unsupported by this handler.
oc-calendar: wDay field cannot be negative

Furthermore, [MS-OXOCAL] Section 2.2.1.41.1 indicates:

   The wDay field is set to indicate
   the occurrence of the day of the week within the month
   (1 to 5, where 5 indicates the final occurrence during
   the month if that day of the week does not occur 5 times).

[rule firstOccurrence] may return negative values according to iCal spec for
recurrent rules iCal 4.8.5.4 Recurrence Rule. For instance, for defining
a timezone whose recurrent rule is done using this rule:

   RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU

This fixes the problem when editing a recurrent appointment in
Outlook were incorrectly shifted when SOGo provides back the event.
oc-calendar: getPidLidClipEnd now follows [MS-OXOCAL]

On recurrent events when there is no end, the time to set is
31 August 4500, 11:59 P.M ([MS-OXOCAL] Section 2.2.1.15)
oc-calendar: Fix PidLidClipStart on recurrent events

The midnight value must be in user's timezone and returned back in UTC.