Conversion utilities: better handling for classes and styles in downcast

[scofalik]

Follow-up to ckeditor/ckeditor5-engine#1274.

Right now there is no way to properly and easily handle setting and removing classes and styles during downcasting.

You can of course use the utilities and (un)set class attribute but this will overwrite already set classes.

[scofalik]

I wonder how configuration should look like for those cases. This concerns only downcastAttributeToAttribute (and also attributeToAttribute two-way-converter).

downcastAttributeToAttribute( {
	model: ...,
	view: {
		key: 'class',
		value: 'myClass' | [ 'myClassA', 'myClassB' ]
	}
} );

downcastAttributeToAttribute( {
	model: ...,
	view: {
		key: 'style',
		value: {
			'font-size': '10px',
			'font-family': 'Arial'
		}
	}
} );

Both of these are compatible with view.ElementDefinition (after slight normalization :)) so can be used in two-way converters. Two small things to note are:

• we are letting to set multiple things (although still one attribute),
• we need to document this (until now it was just String in value).