Provide styling name to extensions

One more fact that can be revealed to the extension is the theme/styling of the surrounding editor. Developers/designers can then choose/provide the correct styling for the extension.

Current values:

• 'classic' (for the styling of the content perspective)
• 'material' (for the styling of the channel manager)

Only for document field extensions is this distinction relevant. May be extended with 'material2' later.

This should be available as ui property 'styling'.