[ADD] web: opacity utility classes

Add new utility classes to handle basic opacity behaviors.
Once migrate to BTS5 these classes should be generated using the
upcoming utility classes API.

Part of the overall v16 SCSS optimization/restyle, task-2704984.

task-2731819

Part-of: odoo/odoo#83108

addons/web/static/src/legacy/scss/ui.scss

@@ -28,6 +28,19 @@
     }
 }
 
+// == Opacity Utility Classes
+//
+// TODO: generate these classes with BTS5's API.
+@each $-opacity-level in (0, 25, 50, 75, 100) {
+    .opacity-#{$-opacity-level} {
+        opacity: $-opacity-level * 1%;
+    }
+
+    .opacity-#{$-opacity-level}-hover:hover {
+        opacity: $-opacity-level * 1%;
+    }
+}
+
 // Allow to change data-toggle="collapse" text according to status
 :not(.collapsed) > .o-collapsed-label,
 .collapsed > .o-not-collapsed-label {