Buttons: Add disabled state (aria-disabled).

Author: Amaury

css/mu.component.button.css

@@ -194,3 +194,10 @@
 .btn-link:hover {
 	color: var(--mu-primary-hover);
 }
+
+/* Disabled */
+.btn[aria-disabled="true"] {
+	opacity: 0.5;
+	pointer-events: none;
+	cursor: default;
+}