Document addButton better

[Chris Graham]

The API docs for addButton are incomplete: ​http://docs.ckeditor.com/#!/api/CKEDITOR.ui

Some properties on 'definition' are not documented, such as the 'icon' property. For a list of properties one can currently find it deeper in the code at plugins/button/plugin.js -- I suggest going through the properties here and deciding which to document alongside addButton.

When documenting the 'icon' property, also mention that this is not the primary way to set icons, which is to use the 'icons' property on the plugin itself and to use matching icon/button names. Explain it is usually better to do it the primary way because then skins are able to override the icons if they wish.