Add Post#hasContent

[rlivsey]

Currently Post#isBlank is false if it has a section, even if there's no content in that section.

I'm currently doing this to check if a post has blank content:

 function editorPostIsBlank(editor) {
  const post = editor.post;
  if (post.isBlank) {
    return true;
  }

  const sections = post.sections;
  if (sections.isEmpty) {
    return true;
  }

  if (sections.length === 1 && sections.head.isBlank) {
    return true;
  }

  return false;
}

[bantic]

This makes sense to me. I think it may also make sense to change post#isBlank to work the way editorPostIsBlank above does, too. Sections currently are "blank" if they have no sections OR if all their sections are blank, so it is inconsistent that posts define "blank"-ness differently.

We currently use post#isBlank to deal with placeholder text (inserting an empty markup section under the cursor when typing in a blank post) and a few other niche cases. Those can likely be fixed up to work with a more consistent version of post#isBlank instead.

Thoughts @mixonic ?

[mixonic]

I think we could introduce isEmpty for the case of a post having no sections, and isBlank for the all-contents-are-blank case. Of course I don't think we need the latter internally? It would just be public API.

Do you want to public API @rlivsey? Seems ok to have both to me.

[bantic]

released in v0.10.7