Provide a more visual template selection process in the post editor

[jameskoster]

Changing the post template can have an enormous impact on how that post appears on the frontend of your site. However the current UI offers no visual feedback whatsoever:

choose.template.mp4

You have to click "Edit" to get a view of how your post will look, but from there it's not possible to change the template until you exit the template editor.

With block templates we can make this a more visual exercise by offering previews as a part of the selection UI.

[jameskoster]

It's possible that a solution for #27847 could facilitate this by making it possible to change template while viewing the layout.

[jameskoster]

It's possible that a solution for #27847 could facilitate this by making it possible to change template while viewing the layout.

I think this is the case. Here's how it could work:

change.template.mp4

Clicking the "New template" option would trigger the UI for selecting a starting point (#41060).

[jameskoster]

In situations where an explicit template exists the dropdown could behave like so: