Block Bindings: Add indicator that an attribute is connected instead of hiding the controls

[SantosGuillamot]

This is part of a broader effort to improve the block bindings UI.

What?

Right now, when an attribute is connected through the bindings, we are hiding the relevant block controls. Instead of that, we could change the UI to indicate that they are actually connected. This is an example of how the image URL modal could look like:

It would complement the inspector panel planned in this other issue.

I believe we can approach each specific block control independently. These are the ones to consider:

• Image URL replacement modal.
• Image link. Specially after supporting all attributes in this pull request.
• Improve the alt and title attributes to follow this format.
• Button URL modal.