Feature: Introduced skipping items when binging collections.

[oskarwrobel]

Suggested merge commit message (convention)

Feature: Introduced skipping items when binging collections. Closes ckeditor/ckeditor5#4990. Closes https://github.com/ckeditor/ckeditor5-ui/issues/92.

[coveralls]

Changes Unknown when pulling 79ee2fc on t/215 into ** on master**.

[coveralls]

Changes Unknown when pulling 19c9905 on t/215 into ** on master**.

[oskarwrobel]

I probably found a bug.

[oskarwrobel]

Fixed (missing index for initial items).

                // Load the initial content of the collection.
                for ( const externalItem of externalCollection ) {
-                       addItem( null, externalItem );
+                       addItem( null, externalItem, externalCollection.getIndex( externalItem ) );
                }

[coveralls]

Changes Unknown when pulling 24db40a on t/215 into ** on master**.