Patch that enables visibility settings in org-mode iCalendar export

[Tobias Schlemmer <keinstein.junior@gmail.com>]

Hi,

for some reasons I need to control the visibility of individual entries
of my caldav calendars. I have implemented a small patch that passes the
`CLASS' propery to the iCalendar export:

https://github.com/keinstein/org-mode/pull/1

I'd like to have this patch integrated in the short term. It is mostly
done by cut and paste from the same file.

For long term I think a configurable framework would be more useful:

• It would be helpful to have a configuration variable that tells which
properties are passed more or less verbatim as iCalendar fields
• It would be helpful to save some/all other fields with some prefix
(X-ORG-MODE-…) in the calendar file/service. The prefix should be
configurable as well as the list of saved properties.
• A mapping between org-mode properties and iCalendar fields could be
helpful. Together with some default action all of the above could be
realized with only two variables.

I'm not familiar enough with Lisp to do the work by myself. So this is a
pure feature request.

Tobias

P.S.: Please add me to CC for all discussions as I'm not on the mailing
list.