CMake: Fix configurations tab bar appearance

A tab bar should span completely above the group that it "tabs", and
have no spacing to it. Otherwise there are funny visual artifacts, and
the visual correspondance is not clear.

Add an option for spacing to LayoutBuilder for that.

Change-Id: I5ca9dc713fff95599c9ec1cf90abe8473bc67c2a
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/utils/layoutbuilder.h

@@ -27,6 +27,8 @@
 
 #include "utils_global.h"
 
+#include "optional.h"
+
 #include <QList>
 #include <QString>
 #include <QVariant>
@@ -97,6 +99,8 @@ public:
 
     ~LayoutBuilder();
 
+    LayoutBuilder &setSpacing(int spacing);
+
     LayoutBuilder &addItem(const LayoutItem &item);
     LayoutBuilder &addItems(const LayoutItems &items);
 
@@ -148,10 +152,12 @@ public:
 protected:
     explicit LayoutBuilder(); // Adds to existing layout.
 
+    QLayout *createLayout() const;
     void doLayout(QWidget *parent);
 
     LayoutItems m_items;
     LayoutType m_layoutType;
+    Utils::optional<int> m_spacing;
     bool m_withMargins = false;
 };