Add custom model for tree view in Database Structure tab

Use a custom model for the tree view in the "Database Structure" tab in
the main window, i.e. change from a QTreeWidget to a QTreeView and do
all the item management stuff manually. This might add some code and
complexity but also offers some more flexibility for us.

src/DbStructureModel.h

@@ -0,0 +1,30 @@
+#ifndef __DBSTRUCTUREMODEL_H__
+#define __DBSTRUCTUREMODEL_H__
+
+#include <QAbstractItemModel>
+class DBBrowserDB;
+class QTreeWidgetItem;
+
+class DbStructureModel : public QAbstractItemModel
+{
+    Q_OBJECT
+
+public:
+    explicit DbStructureModel(QObject* parent = 0);
+    ~DbStructureModel();
+
+    void reloadData(DBBrowserDB* db);
+
+    QVariant data(const QModelIndex& index, int role) const;
+    Qt::ItemFlags flags(const QModelIndex& index) const;
+    QVariant headerData(int section, Qt::Orientation orientation, int role = Qt::DisplayRole) const;
+    QModelIndex index(int row, int column, const QModelIndex& parent = QModelIndex()) const;
+    QModelIndex parent(const QModelIndex& index) const;
+    int rowCount(const QModelIndex& parent = QModelIndex()) const;
+    int columnCount(const QModelIndex& = QModelIndex()) const;
+
+private:
+    QTreeWidgetItem* rootItem;
+};
+
+#endif