[Dashboard] Fixes hidden add panel popover on smaller viewports (elastic#178593)

Author: cqliu1

src/plugins/dashboard/public/dashboard_app/top_nav/editor_menu.scss

@@ -0,0 +1,6 @@
+.dshSolutionToolbar__editorContextMenu {
+  @include euiScrollBar;
+  @include euiOverflowShadow;
+  max-height: 60vh;
+  overflow-y: scroll;
+}

src/plugins/dashboard/public/dashboard_app/top_nav/editor_menu.tsx

@@ -6,6 +6,8 @@
  * Side Public License, v 1.
  */
 
+import './editor_menu.scss';
+
 import React, { useCallback, useEffect, useMemo, useState, useRef } from 'react';
 import {
   EuiBadge,