Blocking ability to drop content in specific container

[Piotrek Koszuliński]

Part of: #11437.

Native drag and drop supports a dragenter event fired when mouse enters drop container. This very useful way to say where user can drop and were not. Though, there's no access to clipboard data at that point so it's impossible to react based on what's dragged.

We can do better inside editor because we'll store dragged content in a global, custom data transfer object. We just need to join all these pieces together.

Plan:

1. On editable's and nested editable's dragenter we should fire editor#dragenter with a reference to container (editable) and our data transfer if exists for this drag.
2. When editor#dragenter is cancelled the native dragenter should be prevented so user is visually notified that content is rejected. This includes general dnd as well block widgets dnd.

[Olek Nowodziński]

cc

[Piotrek Koszuliński]

DUP reported #12137.

[Piotrek Koszuliński]

Let's skip this feature for now. It would be a nice addition to the system, but we need to trim down 4.5.0 a little bit in order to finally release it. Moreover, this functionality would be just a facade, so the same can be achieved with the underlying native features.

[Piotr Jasiun]

This could be a common solution for both real drag and drop and the fake block widget drag and drop, so something more then just a facade, but I totally agree that this is not a good moment for introducing this feature. Fake D&D is not implemented yet and this should be implemented when the fake D&D will be stable.