Add drag and drop image insertion

[thibaudcolas]

Feature. From the old R&D notes.

What is the current behavior?

To insert an image, one needs to use the Image control in the toolbar. This is one of the only controls that does not have a keyboard shortcut for it, so it would be cool to support a more direct drag and drop.

What is the expected behavior?

With the relevant Image entity source properly set up, dropping an image file in the editor creates a new IMAGE entity (and potentially opens relevant editing controls if required).