Disable the Preview button when previews are not possible (or allow previews even when popups are disabled)

[marklchaves]

Describe the feature request

Either disable the Preview button when previews are not possible or show previews when popups are disabled (preferred for consistency).

See the Use case section for more details.

OP: https://wordpress.org/support/topic/preview-broken/

Related issue: #556

Use case

1. If I edit a popup that's disabled, the Preview button is active and when I click it, it brings up a browser window but the popup never appears (because it's disabled).
2. I can save a new popup as a draft. When I do that, the Preview button appears. (Note: There's no enable button because the popup is still not published.) Again, the Preview button is active yet useless and creates confusion.

Example screenshots (if any)

[danieliser]

This one should be fixed in develop branch

[marklchaves]

Tested with:

1. Enabled popup
2. Disabled popup
3. Draft popup

Looks good!