Allow using Views directly in TemplateDefinition

[oleq]

A follow-up of ckeditor/ckeditor5-ui-default#67.

---

To speed–up the process of creating the UI structure, passing views directly into template definition like this should be allowed:

constructor( locale ) {
    super( locale );

    // TODO we still need to specialise those buttons (their type).
    this.okButton = new ButtonView( locale );
    this.cancelButton = new ButtonView( locale );
    this.unlinkButton = new ButtonView( locale );
    this.urlInput = new LabeledInputView( locale );

    this.template = new Template( {
        tag: 'div',
        attributes: [
            class: [ 'ck-link-form' ]
        ],
        children: [
            this.urlInput,
            {
                tag: 'div',
                attributes: {
                    class: [ 'ck-link-form-footer' ]
                },
                children: [
                    this.okButton,
                    this.unlinkButton
                ]
            },
            this.unlinkButton
        ]
    } );
}