Widgets: Offer option to disable preview or a static placeholder

[draganescu]

Some core widgets are only rendered in certain pages, like filters.

They depend on the current page having some products in the loop or some other conditions: for example, Woo's Active Product Filters is only rendered if the shopper has already selected a filter. That means that when they are previewed in the widgets editor, they look empty.

This would mean that we should have a kind of a static placeholder? What would be a nice default for developers and users alike?

Can we offer developers a way to hide the preview button?

[shaunandrews]

I posted an updated design that I think would resolve this: #26179 (comment)

[paaljoachim]

The issue was discussed during a Design triage on slack: https://wordpress.slack.com/archives/C02S78ZAL/p1612888120354200

[noisysocks]

#30889 will close this and the way that a widget can choose to render a static "Preview is not available." placeholder is by rendering no output in its implementation of WP_Widget::widget().