fix(ui5-table-column): fix sticky column overlaps dialogs (#1609)

FIXES: #1602

Author: ilhan007

packages/main/src/themes/TableColumn.css

@@ -29,5 +29,5 @@ th {
 :host([sticky]) th {
 	position: sticky;
 	top: 0;
-	z-index: 999;
+	z-index: 99;
 }

packages/playground/_sass/layout.scss

@@ -95,7 +95,7 @@
   justify-content: space-between;
   align-items: center;
   background-color: #000;
-  z-index: 100;
+  z-index: 10;
 
   .main-content, .main-content-header {
     padding-top: 0;