set_dtend is missing

[tobixen]

There is a get_dtend which is an alias to get_due, but no equivalent set_dtend.

Those convinience-mehods are going to be moved to the icalendar library, but will still live in the caldav library for many years to come due to backward-compatibility. Better have it consistent, even if those methods will soon be considered "legacy`" and "deprecated".

[tobixen]

• set_duration and set_due needs to be moved up to the base class
• methods needs to be rewritten to use DTEND rather than DUE when applicable
• It's not sufficient to do set_dtend = set_due as the named parameter due should be replaced with dtend
• It's needed with test code