restrict Title Block onyl if present on page

[jackahl]

In vanilla volto the title Block is addable when there is no title block present on the page. This behavior is overruled here: https://github.com/eea/volto-slate/blob/master/src/blocks/Title/index.js#L29

I think either the logic from volto should be ported over to here or the blocksconfig should extend the original config and just contain those fields, that have been changed. Original logic in Volto is this: https://github.com/plone/volto/blob/a48d7a2fdc24dad5e07ebe9447b79a66a9a337c8/src/config/Blocks.jsx#L174

[sneridagh]

in fact, the config should only "touch" the overridden parts, not the whole thing.