Added explain functionality.

Uses json format with jsoncpp as a parser. Then show it in a QTreeView.
Shows inclusive/exclusive times like explain.despesz does. Also a similar
coloring scheme as applied.

queryexplainmodel.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <QAbstractItemModel>
+#include <string>
+#include "explaintreemodelitem.h"
+
+class QueryExplainModel : public QAbstractItemModel
+{
+	Q_OBJECT
+public:
+	explicit QueryExplainModel(QObject *parent, std::unique_ptr<ExplainRoot> exp);
+
+	QVariant data(const QModelIndex &index, int role) const override;
+
+//	Qt::ItemFlags flags(const QModelIndex &index) const override;
+
+	QVariant headerData(int section, Qt::Orientation orientation,
+		int role = Qt::DisplayRole) const override;
+
+	QModelIndex index(int row, int column,
+		const QModelIndex &parent = QModelIndex()) const override;
+
+	QModelIndex parent(const QModelIndex &index) const override;
+
+	int rowCount(const QModelIndex &parent = QModelIndex()) const override;
+
+	int columnCount(const QModelIndex &parent = QModelIndex()) const override;
+
+private:
+	std::unique_ptr<ExplainRoot> explain;
+};