Gui: Fix Property View copy bug

The context menu of the property view shows a "Copy" action for
properties with which the value of the property can be copied to the
clipboard.  When multiple properties are selected, this menu item
appears multiple times and in that case it copies the name of one of the
properties.  This commit fixes this.

src/Gui/propertyeditor/PropertyEditor.cpp

@@ -751,6 +751,10 @@ void PropertyEditor::contextMenuEvent(QContextMenuEvent*)
         if (index.column() > 0) {
             continue;
         }
+    }
+
+    // copy value to clipboard
+    if (props.size() == 1) {
         const QVariant valueToCopy = contextIndex.data(Qt::DisplayRole);
         if (valueToCopy.isValid()) {
             QAction* copyAction = menu.addAction(tr("Copy"));