Added support for initializing Collection using a constructor

[mlewand]

Suggested merge commit message (convention)

Feature: Added support for initializing Collection items using a constructor. Closes ckeditor/ckeditor5#6319.

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Additional information

This PR was mostly based on #309 with some notable changes:

• Reverted collection-add-invalid-id and collection-add-item-already-exists error interfaces back to what we have on the master branch (see current vs originally proposed). I don't think there's a strong reason to change this API.
• add() remains unchanged with exception of reusing _getItemIdBeforeAdding().
• Added API docs.
• Added couple more test cases: generates id for items that doesn\'t have it, throws an error when invalid item key is provided, throws an error when items have a duplicated key.

I had some doubts:

• I wonder whether it would be better to do the item initialization as options.items instead of adding initialItemsOrOptions next to options. I believe any xOrY kind of variables are confusing for the API consumer.

Once this PR is merged, the master branch should get merged to the #309 PR so that the diff will get smaller and easier to review.

There's a subpr: ckeditor/ckeditor5-ui#545

[coveralls]

Coverage remained the same at 100.0% when pulling 32866ae on i/6319 into b148e9c on master.

[oleq]

Let's merge it ASAP in the upcoming iteration.