Fix relative z-index of spy panel and filter editor (#13717) (#13736)

* Fixes relative z-index of spy panel and filter editor

* Position sidebar above main content so collapser button remains clickable

Author: Bargs

src/core_plugins/kibana/public/discover/styles/main.less

@@ -8,6 +8,7 @@
   .discover-wrapper {
     padding-right: 0px !important;
     padding-left: @collapser-width;
+    z-index: @discoverWrapperDepth;
   }
 
   .discover-content {

src/ui/public/collapsible_sidebar/collapsible_sidebar.less

@@ -2,7 +2,7 @@
 
 .collapsible-sidebar {
   position: relative;
-  z-index: 1;
+  z-index: @discoverSidebarDepth;
 
   .sidebar-collapser {
     position: absolute;

src/ui/public/styles/variables/depth.less

@@ -1,7 +1,11 @@
 /**
  * 1. The local nav contains tooltips which should pop over the filter bar.
  * 2. The filter and local nav components should always appear above the dashboard grid items.
+ * 3. The filter and local nav components should always appear above the discover content.
+ * 4. The sidebar collapser button should appear above the main Discover content but below the top elements.
  */
-@filterBarDepth: 2; /* 1 */
-@localNavDepth: 3; /* 1 */
+@filterBarDepth: 3; /* 1 */
+@localNavDepth: 4; /* 1 */
 @dashboardGridDepth: 1; /* 2 */
+@discoverWrapperDepth: 1; /* 3 */
+@discoverSidebarDepth: 2; /* 4 */