Integrate Linkit

I tried to add attributes requested in #6. But it turned out that this is simply not possible:

1. Gutenberg's link supports only 4 attributes
   https://github.com/WordPress/gutenberg/blob/fd98542335ce1a2eb0517b9e4f28...

   attributes: {
   	url: 'href',
   	type: 'data-type',
   	id: 'data-id',
   	target: 'target',
   },
   

2. There is no way to extend them
   https://github.com/WordPress/gutenberg/issues/11599

Rewriting LinkControl and core/link is not an option. So I guess @bgilhome already did the maximum of what we can do. Shall we go with his solution?

One additional suggestion: Display entity type and bundle labels in the "type" label.

Currently it is: node, media. etc.

It can be: Content: Page, Media: PDF, etc.

It would be especially handy because Linkit can group suggestions by bundles.

Slightly overhauled the patch to use the linkit services instead the controller to fetch the suggestions.
And using ContainerInjectionInterface to inject these services.
Output format etc. stays the same.