Fix the extend-template-render warning

[Reinmar]

UI lib introduced a warning about extending already rendered templates and it turns out that the image feature logs a lot of them:

https://travis-ci.org/ckeditor/ckeditor5-image/builds/195804693#L1452

[Reinmar]

The problem is here:

		const panel = this._panel = new ImageBalloonPanel( editor );
		const promises = [];
		const toolbar = new ToolbarView();

		// Add CSS class to the panel.
		Template.extend( panel.template, {
			attributes: {
				class: [
					'ck-toolbar__container'
				]
			}
		} );

And... well, ping @oleq for a solution ;>

[oleq]

The solution is: extend first, render next. And by "render", I mean "access View#element", which renders the element on demand.

[Reinmar]

Yeah, but what's accessing #element in this case? It must be ImageBalloonPanel itself.

[Reinmar]

ImageBalloonPanelView#constructor:

		// Let the focusTracker know about new focusable UI element.
		editor.ui.focusTracker.add( this.element );

This would need to be moved to init().