Shred the vobject-dependency

[tobixen]

Many contributors have said that the icalendar library is superior to the vobjects library. There have been several independent pull-requests submitted throughout the time aiming to replace vobjects with icalendar - but in the name of backward compatibility I've rejected it, instead I made support for both libraries.

The vobject should be made into an optional dependency, and if nobody uses the vobject code, then we should consider to shred all the vobject code at some point - perhaps deprecation warnings from 2.0 and removal in 3.0

[tobixen]

vobject is a live and active project, so it shouldn't be shredded. But optional dependency with late imports ... that's probably a good idea.