Allow features to pre-configure their toolbars

[Reinmar]

In CKEditor 4 features were able to define to which toolbar group a button should go and what's its order. Additionally, there was a predefined configuration of the toolbar layout. The developer could've added a plugin to the editor and its buttons were automatically appearing in the toolbar in predefined places.

This also allowed the developer to only change the configuration of toolbar groups, without reconfiguring the whole editor.

Another important aspect of toolbar groups based configuration was accessibility. When cycling the toolbar groups with the Tab key the user would hear the group name. This simplified navigation through the toolbar if it had many buttons.

In CKEditor 5 things are more complex and simpler at the same time:

• We can have multiple toolbars – some are introduced by features (image toolbar), some by creators (the main toolbar, the floating toolbar, etc.). So there's no one configuration which could be defined in some specific place.
• The toolbars are much simpler (at least for now :D). There are just a couple of buttons and there should never be more, so perhaps the groups are not needed anyway. We can also handle the "big toolbar" problem differently in the future – e.g. by introducing tabs or grouping buttons in dropdowns, etc.

So, for now, we'll definitely avoid working on this problem. We don't know how things will look in the future and we can live with some simple alternatives (see https://github.com/ckeditor/ckeditor5-ui/issues/155#issuecomment-282300881).

However, we may start gathering some ideas. Hence, this ticket.

[Reinmar]

Lack of default automatic configurations causes issues when configuring prebuilt editors: #526.

[CKEditorBot]

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[CKEditorBot]

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