Fix losing Focus on moving diagram position with keyboard

Due to the changes made in the commit "Add highlight current page in
ProjectView", there is a problem when moving diagrams in the ProjectView
using the keyboard. The diagrams lose focus after being moved.
The cause is: The DiagramItem loses its selection before the move
function is executed.

The code has been adjusted.

sources/genericpanel.h

@@ -171,6 +171,9 @@ class GenericPanel : public QTreeWidget {
 					 const QString &);
 	
 	// various other methods
+	public:
+	void setSelectedItem(QTreeWidgetItem *selectedItem);
+
 	protected:
 	virtual QString defaultText(QET::ItemType);
 	virtual QIcon   defaultIcon(QET::ItemType);
@@ -222,5 +225,7 @@ class GenericPanel : public QTreeWidget {
 		representing a title block template
 	*/
 	QHash<TitleBlockTemplateLocation, QTreeWidgetItem *> tb_templates_;
+
+	QTreeWidgetItem *m_selected_item = nullptr;
 };
 #endif