Add FlatGroupUi

This solves the problem of trying to show/hide indexes that do not even
exist on the DOM.

There are no tests for this due to the complexity of the parent; refactoring
is needed. :(

* src/ui/group/FlatGroupUi: Added

src/ui/group/FlatGroupUi.js

@@ -0,0 +1,68 @@
+/**
+ * Group UI rendering only the first index
+ *
+ *  Copyright (C) 2016 LoVullo Associates, Inc.
+ *
+ *  This file is part of liza.
+ *
+ *  liza is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+var Class   = require( 'easejs' ).Class,
+    GroupUi = require( './GroupUi' );
+
+/**
+ * Renders only the first index of fields in a flat group that does not
+ * support any means of adding or removing indexes.
+ */
+module.exports = Class( 'FlatGroupUi' )
+    .extend( GroupUi,
+{
+    /**
+     * Permit adding only a single index
+     *
+     * @param {number} index index that has been added
+     *
+     * @return {GroupUi} self
+     */
+    'protected override addIndex': function( index )
+    {
+        if ( index > 0 )
+        {
+            return this;
+        }
+
+        return this.__super( index );
+    },
+
+
+    /**
+     * Permit removing only the first index
+     *
+     * This follows from #addIndex, since only one will ever exist.
+     *
+     * @param {number} index index that has been removed
+     *
+     * @return {GroupUi} self
+     */
+    'protected override removeIndex': function( index )
+    {
+        if ( index > 0 )
+        {
+            return this;
+        }
+
+        return this.__super( index );
+    }
+} );