Utils: Add an 'If' LayoutItem

Change-Id: I700e28d03b6dc39e96fa969152f236760989bdb7
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/libs/utils/layoutbuilder.cpp

@@ -819,6 +819,13 @@ Tab::Tab(const QString &tabName, const LayoutItem &item)
     };
 }
 
+// Special If
+
+If::If(bool condition, const LayoutItems &items, const LayoutItems &other)
+{
+    subItems.append(condition ? items : other);
+}
+
 // Special Application
 
 Application::Application(std::initializer_list<LayoutItem> items)