Implement font family editing plugin

[jodator]

Requirements similar to font size editing plugin in term of output and configuration.

Probably other requirements will be addes if any talks in #2275 will happen.

[jodator]

Font family configuration we have to define default set of font families.

Full flavor config with ConverterDefinition:

config.fontFamily = {
    items: [
        'default',
        {
            label: 'Arial',
            model: 'arial',
            view: {
                    // Used also to define ui preview in dropdown?
                    styles: {
                        font-family: 'Arial, Helvetica, sans-serif'
                    }
            }
            // others
        }
    ]
}

Shorthand config

It also makes sense to define config just by defining short-hand definition. Taken CKEditor 4 as an example I'd propose something similar:

config.fontFamily = {
	items: [
		'default',
		'Arial, Helvetica, sans-serif',
		'Courier New, Courier, monospace',
		'Trebuchet MS, Helvetica, sans-serif',
		'Verdana, Geneva, sans-serif'
	]
}

it would create full-flavor configuration and will take first font name as label as well as model value.

Defaults

For default values we migth go with something as in CKEditor 4 - but maybe shorter? At least we migth drop Comic Sans there for the best of humanity.

const defaults = [
	'Arial, Helvetica, sans-serif',
	'Courier New, Courier, monospace',
	'Georgia, serif',
	'Lucida Sans Unicode, Lucida Grande, sans-serif',
	'Tahoma, Geneva, sans-serif',
	'Times New Roman, Times, serif',
	'Trebuchet MS, Helvetica, sans-serif',
	'Verdana, Geneva, sans-serif'
]

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Also I wonder if we could create a bit more "semantic" shorthand configuration / presets for defining Font Family view with classes.

Another: does pushing default makes sense? shouldn't we always put "default" item in dropdown?