Write a valid VTIMEZONE on Windows CE

On Systems that do not provide a valid database about
Timezone transitions the VTIMEZONE entry of the ical
data would only contain the Tzid.

For WinCE there is now a fallback to write the current Timezone
information in the entry. This works since the gui on WinCE
does not offer the user to create an Event in a different Timezone.

I've added the code here in kcalcore instead of KTimeZone because
this is tailored specifically to what the ical format requires
and can not be generalized to other timezones then the current
Zone.