Reorganize codes

Reorganize all parts of codes;
Fix a bug that index page color is not correct when initialized.

src/flowlayout.cpp

@@ -117,14 +117,14 @@ QLayoutItem *FlowLayout::takeAt(int index)
     if (index >= 0 && index < itemList.size())
         return itemList.takeAt(index);
     else
-        return 0;
+        return nullptr;
 }
 //! [5]
 
 //! [6]
 Qt::Orientations FlowLayout::expandingDirections() const
 {
-    return 0;
+    return nullptr;
 }
 //! [6]
 
@@ -215,7 +215,7 @@ int FlowLayout::smartSpacing(QStyle::PixelMetric pm) const
         return -1;
     } else if (parent->isWidgetType()) {
         QWidget *pw = static_cast<QWidget *>(parent);
-        return pw->style()->pixelMetric(pm, 0, pw);
+        return pw->style()->pixelMetric(pm, nullptr, pw);
     } else {
         return static_cast<QLayout *>(parent)->spacing();
     }