Exposes Contexts to allow custom plugins

[thomasouvre]

Is your feature request related to a problem? Please describe.
I want to change the alpha value of the colors selected from the pickers (text and background colors. I need the "transparent" color too.
I see I cannot do that in the current BackgroundColorPicker component. So I tried to made a custom one, but I need the ToolbarContext and I can't import it.

Describe the solution you'd like
We can allow the editing of the alpha part in the ColorPicker component, but we can open the toolbar plugin system to allow any custom component and let the developer doing is own customizations.

Describe alternatives you've considered
I made a JSON editor of the state and edit the state manually. But end users can't do that...

Thanks for the project :)