style(action-pane): clip heading label (openscd#528)

this prevents long headings from extending across the right pane border on small screens

Author: JakobVogelsang

src/action-pane.ts

@@ -152,7 +152,7 @@ export class ActionPane extends LitElement {
       color: var(--mdc-theme-on-surface);
       font-family: 'Roboto', sans-serif;
       font-weight: 300;
-      overflow: visible;
+      overflow: clip visible;
       white-space: nowrap;
       text-overflow: ellipsis;
       margin: 0px;