OO-531: fix z-index issue in tree which hide the close/open link

src/main/webapp/static/themes/openolat/all/modules/_nav.scss

@@ -185,6 +185,10 @@ div.b_tree {
 					position:absolute; top:0px; left: 1px + ($i * $tree_padding_steps_px); 
 					z-index:9; 
 				}
+				
+				.b_tree_level_open.b_tree_oc_l#{$i}, .b_tree_level_close.b_tree_oc_l#{$i} {
+					z-index: 10;
+				}
 			}
 
 			@for $i from 0 through 11 {

src/main/webapp/static/themes/openolatexample/all/content.css

-@import url("../../openolat/all/content.css");body{font-family:Courier;font-size:90%;color:#444}h1,h2,h3,h4,h5,fieldset legend{text-shadow:0 3px 3px rgba(0,0,0,0.4);-moz-text-shadow:0 3px 3px rgba(0,0,0,0.4);font-family:Comic Sans,Comic Sans MS,cursive}a,a:visited{text-decoration:none;color:red}a:hover{text-decoration:underline;color:red}
+@import url("../../openolat/all/content.css");body{font-family:Courier;font-size:90%;color:#444}h1,h2,h3,h4,h5,fieldset legend{text-shadow:0 3px 3px rgba(0, 0, 0, 0.4);-moz-text-shadow:0 3px 3px rgba(0, 0, 0, 0.4);font-family:Comic Sans, Comic Sans MS, cursive}a,a:visited{text-decoration:none;color:red}a:hover{text-decoration:underline;color:red}