gfse: example-based: some usability improvements

Instead of showing nothing, show disabled buttons and some expanatory text to
provide guidance for users getting started with example-based grammar writing.

src/www/gfse/editor.css

@@ -52,7 +52,7 @@ div.template:hover
 .edit { color: orange; }
 .delete { color: red; }
 .error_message,.inError { color: red; }
-.template, .template .sep { color: #999; }
+.template, .template .sep, .unimportant { color: #999; }
 form { display: inline-block; }
 
 table.tabs {