Icon on WP 4+

I noticed that on WP 4.9 the “+” icons on widget title bar not appear.

That happens because the CSS rules of you plugin:

.oomph-cloneable a.clone-widget::after {
content: "\f132";
}

The following change should fix it:

.oomph-cloneable a.clone-widget::after {
content: "\f132"; font-family: dashicons;
}