Layout: Add metabox placeholder shell

[aduth]

Closes #2760
Closes #2322
Related: #2583

Closed	Opened

While development of metaboxes continues in #2583, this pull request seeks to add a static content shell for representing the container in which meta fields will be rendered. In doing so, it tackles styling issues with the current layout where content does not occupy the full height of the viewport, complicating panels pinned to the bottom, and disrupting "click outside" behavior for the currently selected block.

Testing instructions:

1. Navigate to Gutenberg > Demo or Gutenberg > New Post
2. Note that an "Extended Settings" toggle heading is shown at the bottom of the page
3. Toggle the Extended Settings panel
4. Note that the panel expands upward, and that it is still possible to reach all content of the post while the panel is expanded

[codecov]

Codecov Report

Merging #2800 into master will decrease coverage by 0.08%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2800      +/-   ##
==========================================
- Coverage   33.82%   33.73%   -0.09%     
==========================================
  Files         189      190       +1     
  Lines        5661     5676      +15     
  Branches      988      991       +3     
==========================================
  Hits         1915     1915              
- Misses       3171     3183      +12     
- Partials      575      578       +3

Impacted Files	Coverage Δ
editor/meta-boxes/index.js	0% <0%> (ø)
editor/layout/index.js	0% <0%> (ø)	⬆️

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[mtias]

🚢 it

[jasmussen]

💥

[rosswintle]

Not sure if this is the right place to comment. And apologies if this is all covered elsewhere, but I wanted to give some feedback on seeing this:

a) Meta boxes currently have several places that they can live: in the sidebar, below post content with different priorities and contexts
b) I also have cases where I've improved the editing experience for my users by having meta boxes above or below the title because this fits with their content editing flow.
c) I really don't like the "Extended settings" title. For some editing workflows the information in meta boxes is actually critical, core content/settings, not something optional/added-on/extended. Is this editable? Can developers add additional sections of their own like this?

[mrwweb]

+1 for everything @rosswintle is saying. I'll add that plenty of interfaces have required meta fields which almost certainly shouldn't be kept in a collapsed-by-default area.

[youknowriad]

@rosswintle @mrwweb These concerns are legitimate and have already been raised in #952

For the first iteration, we'll probably keep the collapsed state but have multiple areas.
There are some good design proposals dropping the expanding area (for the content area) and replacing them with "separators" might be good as v2 #952 (comment)

Feel free to discuss this more in the metaboxes issue