iCalendar import: update the object too when "last-modified" was not changed

According to http://tools.ietf.org/search/rfc5545#section-3.8.7.3, the "last-modified" property contains the date/time of the last change in the calendar store (server side).  Thus, the client is not obliged to update it.

When updating the event in e.g. Apple iCal, the last-modified property is not updated (since it is the job of the server to do this) and this results in the change not being stored when re-importing the event.
(Background: I'm experimenting with letting Plone act as a CalDAV server).

This change still makes sure that we don't have the "lost update" problem (e.g. two users make modifications on the same base version and the second overrides the changes of the first one) by making sure that the "last-modified" date of the import is not older than that of the currently stored event.

@thet Since you're the author of the implementation strategies (b746421227c2c661e05c9ad7d5ab44a595f62c9c): does this change break other use cases of the iCalendar export / import?  I'm not sure what all the use cases are :wink: