When themes don't support widgets the editor looks broken

[draganescu]

In the classic widgets editor when a theme has no widget support we see a message. In the new widgets editor we get one widget area with inactive widgets and no message.

To reproduce:

1. Have Gutenberg plugin installed
2. Install a no widget theme such as Fonts
3. Go to Appearance > Widgets notice there are no widget areas, except "Inactive widgets"
4. Manually go to /wp-admin/widgets.php, notice the message

Screenshots:

Classic widgets editor

Block-based widgets editor

[mapk]

I just tested this with the Fonts theme. Is there a reason we show the Widgets menu item in the wp-admin menu if a theme does not support widget areas? Can we just hide that menu option?

Example of Fonts theme & Widgets menu option still visible

If there remains a reason to keep this option available even if the theme doesn't support widgets, we might be able to make use of the existing notifications in Gutenberg.

Existing notifications

Mock up of proposed design

[noisysocks]

Noting that WordPress/wordpress-develop#1308 contains a fix for this.