Use placeholder only for last item

refs #4535

- as discussed in the meeting on 1st February ;)
- changed the fake-placeholder to only operate on the last item, this way it feels right, I think

core/client/components/gh-navitem-url-input.js

@@ -16,7 +16,7 @@ var NavItemUrlInputComponent = Ember.TextField.extend({
     }),
 
     fakePlaceholder: Ember.computed('isBaseUrl', 'hasFocus', function () {
-        return this.get('isBaseUrl') && !this.get('hasFocus');
+        return this.get('isBaseUrl') && this.get('last') && !this.get('hasFocus');
     }),
 
     isRelative: Ember.computed('value', function () {

core/client/templates/components/gh-navitem.hbs

@@ -8,7 +8,7 @@
         {{gh-trim-focus-input focus=navItem.last placeholder="Label" value=navItem.label}}
     </span>
     <span class="navigation-item-url">
-        {{gh-navitem-url-input baseUrl=baseUrl url=navItem.url change="updateUrl"}}
+        {{gh-navitem-url-input baseUrl=baseUrl url=navItem.url last=navItem.last change="updateUrl"}}
     </span>
 </div>
 <span class="navigation-item-action">