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)

src/libs/utils/layoutbuilder.h

@@ -141,6 +141,12 @@ public:
     Tab(const QString &tabName, const LayoutItem &item);
 };
 
+class QTCREATOR_UTILS_EXPORT If : public LayoutItem
+{
+public:
+    If(bool condition, const LayoutItems &item, const LayoutItems &other = {});
+};
+
 class QTCREATOR_UTILS_EXPORT Group : public LayoutItem
 {
 public:

tests/manual/layoutbuilder/demo/main.cpp

@@ -17,10 +17,18 @@ int main(int argc, char *argv[])
         title("Hello World"),
 
         Column {
+            If { false, {
                     TextEdit {
                         id(textId),
                         text("Hallo")
                     },
+                }, {
+                    TextEdit {
+                        id(textId),
+                        text("Och noe")
+                    },
+                }
+            },
 
             Row {
                 SpinBox {