Refactor Offcanvas to programmatically retrieve the last inserted block

[getdave]

Currently the appender in the offcanvas uses a callback approach to retrieve the ID of the block that has just been inserted.

This is confusing, requires a lot of prop drilling and breaks the Redux paradigm.

In #46531 I'm proposing a selector to retrieve the ID of the last inserted block. Once that lands we can refactor the offcanvas implementation to use the selector and remove all the callback stuff.

[getdave]

This has been implemented.