Try: State: Allow for save blocking

[aduth]

Closes #2978

This pull request seeks to explore an option for resolving autosave clobbering of Text mode content. It introduces a new save blocking mechanism which prevents saves from occurring while active. For the moment, this is implemented while changing the Text mode textarea. In the future, it could potentially be used as a mechanism for preventing autosave from occurring when an upload is in progress.

Design notes:

One major drawback of this approach is that it is not intuitive for a user to save their changes when editing within the Text mode. Further, it may be desirable for autosave to occur while editing the Text mode textarea, as long as it takes edits from the Text mode as precedent over the pending Visual mode changes. To achieve the latter, this would require some signalling from the editor to the Text mode to indicate that a save is about to occur.

Testing instructions:

Repeat testing instructions from #2978, verifying that autosave does not kick in when making edits in the textarea after switching to Text mode.

[mcsf]

With #5755 merged, can we close this one?

[aduth]

We'll need something along these lines for the other issues, like those noted in the original issue:

it could potentially be used as a mechanism for preventing autosave from occurring when an upload is in progress.

Perhaps also as part of an API toward "required" attributes of a block for save. Though this would require some UX design consideration before a technical proposal can be made.

I'll create an issue at least for "autosave while uploading", and close this one.