Add css class for toogle preview icon

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

core/css/icons.scss

@@ -369,6 +369,10 @@ img, object, video, button, textarea, input, select {
 	background-image: url('../img/actions/toggle.svg?v=1');
 }
 
+.icon-toggle-pictures {
+	background-image: url('../img/actions/toggle-pictures.svg?v=1');
+}
+
 .icon-triangle-e {
 	background-image: url('../img/actions/triangle-e.svg?v=1');
 }