hide alertify when you are in presentation mode

www/slide/app-slide.less

@@ -176,7 +176,7 @@ div#cp-app-slide-modal {
         position: fixed;
         top: 0px;
         left: 0px;
-        z-index: 100000;
+        z-index: 1000000; // one order of magnitude higher than alertify
         height: 100vh;
         width: 100%;
     }
@@ -329,4 +329,4 @@ p {
 
 pre.cp-slide-css-error {
     color: white;
-}
+}