Implement slave contact groups — label transfer, terminal assignment and UI fixes

sources/ui/linksingleelementwidget.h

@@ -22,6 +22,8 @@
 #include "abstractelementpropertieseditorwidget.h"
 
 #include <QHash>
+#include <QDialog>
+#include <QComboBox>
 
 class QTreeWidgetItem;
 class Element;
@@ -101,6 +103,8 @@ class LinkSingleElementWidget : public AbstractElementPropertiesEditorWidget
 	Element *m_showed_element = nullptr,
 			*m_element_to_link = nullptr;
 
+	int m_pending_group_index = -1;
+
 	QMenu *m_context_menu{nullptr};
 	QAction *m_link_action{nullptr},
 			*m_show_qtwi{nullptr},