Clean-up and refactoring

As we are breaking backwards compatibility anyway, we should try to clean the code base.

Some ideas:

• there are lots of different ways to create events at the moment, we should aggregate and only expose as few as possible
• better naming convention for different types of events, we currently have
  • strings in icalendar format (starting with BEGIN:VCALENDAR), which are often names ics or ical (and probably some other things) -> suggestion: ics (ical string)
  • icalendar.Vevents() -> suggestion: vevent
  • List[icalendar.Vevents() - often called events_list
  • our own khalendar.event.Events() -> suggestion: event
  • urwid widgets representing events (several different types I believe)
• rename backend to cache
• rename CalendarCollection.new() to insert()

see #1160

Review the remaining unchecked naming-convention and backend-to-cache items, using the event types described in this issue across the Python codebase. Check the related discussion in #1160 and identify the affected icalendar, khalendar, and urwid references. Done means the agreed names and backend terminology are applied consistently.