Introduce retainable document property

We went with option number 1 from adouma's previous comment in RD-5363 ( Keep draft epic ) ) and the "transferable" concept has been added to the document workflow.

Add an additional property or mixin: retainable. A retainable document is a document that can accumulate draft changes by repeated "keep draft" actions.

The main use case is the following situations:

1. In the content perspective when a tab is being closed and in the visual editor when the right side panel is being closed: only for a non-retainable document the user should be presented with the option to cancel, discard or save the changes.
2. Give the user feedback that there are draft changes as an effect of 1 or multiple "keep draft" actions ( feedback messages and icons ).

To implement it without the "retainable" concept means monitoring the "keep draft", "save" and "discard" actions in the content editor and the visual editor. Showing an icon in the document tree becomes draconic.

In my opinion it will increase the code quality, decrease the amount of code and take less time to implement.