[Feature] Multiple toolbox items (new solution)

This PR is a proposal solution to the #79.

I would suggest using data overrides instead of config overrides in the toolbox property.

It allows:

to get rid of activeToolboxEntry getter (it could be matched by current block data)
to not to force developers for using config
to allow developers to override any Block properties via multiple toolbox items. For example, set predefined texts and so on.
the data is supposed to be supported in rendering already

editor-js / header