Toolbar creator should support item priorities

[Reinmar]

Explained in https://github.com/ckeditor/ckeditor5-ui/issues/154#issuecomment-276922081.

Currently, the image plugins (ImageStyle and ImageAlternativeText) add their buttons to the config.image.defaultToolbar array. So if ImageStyle is loaded first, its buttons will be before the ImageAlternativeText button.

It'd be good to decouple this – the order in which plugins are loaded should have no effect on the order of buttons in the toolbar. Hence, we need to prioritise them. After all plugins are loaded, the configuration should look like this:

config.image.defaultToolbar = [
  { name: 'imageAlternativeText', order: 10 },
  { name: 'imageStyleFull', order: 0 },
  { name: 'imageStyleSide', order: 0 }
];

This should render the image alt text button after image style buttons.

Note: I proposed order instead of priority to avoid all those unclarities whether higher priority number means displaying earlier or later.

[Reinmar]

PS. This means that there will need to be a tool which converts toolbar configuration to toolbar items. We're doing this manually now for every toolbar AFAIK.

[Reinmar]

OK, we're not going to work on this. We decided to preconfigure the toolbars only in builds. The features will not be able to deal with this for now.

See more in #408.