css: fix toolbar overlay so it cover only toolbar and not the whole screen

This allows to copy & paste the pad text even when disconnected.

src/static/css/pad/toolbar.css

@@ -1,5 +1,6 @@
 .toolbar {
   display: none;
+  position: relative;
   background-color: #f4f4f4;
   color: #666;
   border-bottom: 1px solid #ccc;

src/static/js/pad_modals.js

@@ -44,6 +44,7 @@ var padmodals = (function()
       });
     },
     showOverlay: function() {
+      // Prevent the user to interact with the toolbar. Useful when user is disconnected for example
       $("#toolbar-overlay").show();
     },
     hideOverlay: function() {