Split Editor.build into Editor.builtinPlugins and Editor.defaultConfig

[Reinmar]

Part of #1038.

Let's split this single Editor.build property to make it less confusing. We've seen that people tried to edit it like this:

ClassicEditor.build = {
	plugins: [
		EssentialsPlugin,
		UploadadapterPlugin,
		AutoformatPlugin,
		BoldPlugin,
		ItalicPlugin,
		BlockquotePlugin,
		EasyimagePlugin,
		HeadingPlugin,
		ImagePlugin,
		ImagecaptionPlugin,
		ImagestylePlugin,
		ImagetoolbarPlugin,
		ImageuploadPlugin,
		LinkPlugin,
		ListPlugin,
		ParagraphPlugin
	],
	// WRONG PLACE!
	image: {
		toolbar: [ ... ]
        },
	config: {
		toolbar: { ... },
		// IT SHOULD BE HERE!
		image: {
			toolbar: [
				'imageStyle:full',
				'imageStyle:side',
				'|',
				'imageTextAlternative'
			]
		},
		language: 'en'
	}
};

Two separate properties will be safer:

ClassicEditor.builtinPlugins = [
	// ...
];

ClassicEditor.defaultConfig = {
	toolbar: { ... },
	image: {
		toolbar: [
			'imageStyle:full',
			'imageStyle:side',
			'|',
			'imageTextAlternative'
		]
	},
	language: 'en'
};