Honour SOURCE property on importing .ics files
When evolution 'imports' ics (iCalendar) file (e.g. after the user clicks on an https://….ics link), and the file has SOURCE property in the VCALENDAR object, Evolution shall ask the user, whether
- she wants to do one time import of the data, or
- create a new (on the web) calendar for that SOURCE in Evolution (subscribe)