Clone entities on paste

[thibaudcolas]

Feature: entities should be cloned when the same entity is referenced in multiple ranges. Each entity should point to a single range.

What is the current behavior?

When pasting from one editor to itself, entity ranges point to the same entity. This is bad, because updating the entity data will affect unexpected ranges. For example, changing a link's target URL should only affect the link where the edit operation is happening, not other links in the document.

What is the expected behavior?

On paste, entities should be cloned so each range has its own entity. Subsequently updating one entity (say a LINK) shouldn't change the data of entities with other ranges.