Gui: Fix copying elements in the tree view by holding CTRL key

In older versions it was possible to copy elements with drag and drop by holding the CTRL key. Since v0.20 or v0.21 this wasn't possible any
more so that elements are always moved.

Note: Copying is only allowed for elements that have a parent object.

src/Gui/Tree.cpp

@@ -620,7 +620,6 @@ TreeWidget::TreeWidget(const char* name, QWidget* parent)
 
     this->setDragEnabled(true);
     this->setAcceptDrops(true);
-    this->setDragDropMode(QTreeWidget::InternalMove);
     this->setColumnCount(3);
     this->setItemDelegate(new TreeWidgetItemDelegate(this));
     this->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);