Allow copying source from other calendars

[niccokunzmann]

When we have a federated network like the Makers of brandenburg, we have participants who have their own calendars. I would like to keep the maintenance of the local in once place and the joined calendar to copy just the sources and their configuration from their spec files.

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[niccokunzmann]

(1)

{
    "copy" : {
        "urls" : [
            "..."
        ],
        "attributes" : [ "url" ]
}

or (2) before we over.engineer:

{
    "copyUrlsFrom" : ["..."]
}

or (3) if we allow different types of inputs as URLs

{
    "url" : "specification.json"
}

(1) would make it difficult to again parse the different parts of other urls but allow re-using design. (2) and (3) would not solve the design use case in case you would like use a uniform design on different platforms but very well the use-case to have a federated structure where people can embed different sources and share them with other calendars.

[niccokunzmann]

see also #323