fix(style): raise dropdown z-index above sticky header cells

Why this change:
`.dt-dropdown__list` had z-index: 1 which placed it behind
`.dt-cell--sticky-top` (z-index: 4), causing the column action
 menu to render underneath sticky header cells like the checkbox.

Author: KerollesFathy

src/style.css

@@ -235,7 +235,7 @@
     &__list {
         position: fixed;
         min-width: 8rem;
-        z-index: 1;
+        z-index: 10;
         cursor: pointer;
         background-color: var(--dt-cell-bg);
         border-radius: var(--dt-border-radius);