Replace "Template Parts" list with area-based alternative

[jameskoster]

Part of #36597

The Template Parts list works well:

However, "Template Parts" is a fairly technical term that we arguably shouldn't be surfacing unless absolutely necessary. Since template parts can be associated with specific areas, we might use those to refine the navigation:

The main drawback here is that "general" template parts would only be accessible through templates directly.

cc @kellychoffman

[carolinan]

I like the simplicity of showing only header and footer, it looks nice.

The problem I see is user created template parts.
"General" is the default area for user created template parts, and I don't think it is obvious that:

• There is something called an area
• How to change the area

I think that is why the template part area setting is under the Advanced panel.

If the general template parts are only accessible through templates, how are they deleted? It is not enough to only select and delete the block in place in the editor.
If I create a template part, I need to be able to find it again, so not having it listed is a major drawback, imagine just testing things, and it being added as clutter in the database.

[jameskoster]

I suppose we could still have a "Template Parts" menu item, which is where general template parts would live.

(ignore the icons)

[talldan]

(ignore the icons)

The icons look really nice though 😄

[jameskoster]

We can try adding the icons too, but I suspect that would involve edits to the underlying component and so might be better handled separately?

[talldan]

Here's a PR that tackles adding the icons - #36893

[ribaricplusplus]

Should this still be done or should it be closed? I see that the main tracking issue #36597 is closed.

[jameskoster]

I think there's a decent argument for this change, if it's simple enough to implement. Not a strong opinion though, because it's unclear if this particular UI will persist in the long term (see #39885).

[annezazu]

Closing this out as the interface has evolved immensely, including a "Patterns" section with template parts listed. What remains for consideration is this issue around emphasizing header/footer more: #50958