Editable: sync state on input, signal undo levelling

[ellatrix]

Description

See #2758.

This PR attempts to let Editable continuously sync state on the input event, and signal for the creation of undo levels when it is triggered by TinyMCE. This keeps the logic for creating undo levels in TinyMCE in use, though not the data.

This is very much a work in progress, but I'm creating a PR now for further discussion.

• Ideally there need to be several improvement to the current undo reducer first as it seems to create too many undo levels and there are several bugs. See e.g. Auto save wipes undo levels #2915, Undo level present on load #2916, Automatic status change triggers undo level #2931 and Undo level created for every character typed in textarea #2932.
• This PR adds the selection reducer as an undoable reducer, not because it should trigger and undo level, but because we need the selection data to set back the selection (both the block and in editable). But do we even want to reset the selection, or do we just want to blur everything? Resetting seems smoother, but more complex I guess.

@aduth would love to hear your general thoughts on this.

[gziolo]

@iseulde does it also try to solve #2758 (Editable: Invoke onChange on every input), which we need to address before we can merge #2863 (Keyboard Shortcuts: Bind Cmd/Ctrl+S as save).

Some context about what's blocking #2758:

This is a nice usability improvement, but I think I'd want to wait until after we resolve editor content not being kept in sync before moving to merge this, since as you note it can be confusing that Save is not available until moving focus.

[ellatrix]

Yep, sorry, forgot to link #2758.

[aduth]

Resetting seems smoother, but more complex I guess.

Yeah, I would very much prefer we not have the additional settings argument passed to onChange, just the next value, or at least obscuring this from the concern of the block implementer. I'm having a hard time following the need for this, and will plan to circle back around with a fresher mind tomorrow.

[ellatrix]

Closing in favour of #4956.