show sidebar label on collapsed sidebar SPR-957

This removes the base z-indexes as they seem not to be needed (source
order defines initial heights) and interfered with the sidbar functionality.

Removing it was suggested in SPR-899 as well

css/area_main-sidebar-nav.less

@@ -84,8 +84,14 @@
         }
     }
 
+    // the wrapper around the toggle to reserve space
+    div.nav {
+        height: @icon-size + @margin-small;
+        border: 1px solid transparent;
+    }
+
     // the toggle element
-    a.nav {
+    div.nav a {
         .display-flex();
         .align-items();
 
@@ -156,6 +162,11 @@
             color: @ini_nav_menu_hover_color;
             text-decoration: none;
 
+            // always show label, even with collapsed sidebar
+            position: absolute;
+            z-index: 100;
+            width: 100%;
+
             span.ico {
                 border-color: inherit;
 
@@ -284,7 +295,6 @@
     }
 }
 
-
 /* + + + + + + + + + + + + + + + + + + + + + + + + + + */
 /* max-width: 1439px */
 

css/base_structure.less

@@ -73,16 +73,7 @@
         z-index: 900;
     }
 
-    .tools .row > .col-xs-12 {
-        z-index: 1;
-    }
-
-    .header .row > .col-xs-12 {
-        z-index: 2;
-    }
-
     .content .row > .col-xs-12 {
-        z-index: 3;
         border-radius: 0 @ini_default_border_radius @fix_border-radius @fix_border-radius; // @ini_default_border_radius vs. @fix_border-radius
     }