Reusable blocks have moved to a multi entity save flow

[johngodley]

A change in Gutenberg 9.7 has meant that inline editing of reusable blocks is now possible. Changes are saved outside of the normal saving, and are part of a 'multi entity save flow'.

The P2 Editor needs to be aware of this and probably also perform what is needed for this multi entity save flow.

WordPress/gutenberg#27887

See Automattic/p2editor#507

[johngodley]

This is quite tricky and will need some thought. The P2 Editor doesn't use the core/editor store for anything, and that is where reusable block changes are stored.