If we want to allow any plugins (other than the data manager plugin itself) to contribute with actions, we have to pay attention on the way the icon is retrieved. I have noted that retrieving icon by doing QIcon(iconPath) works only when the icon is a resource of the same plugin. I suggest using something like that:

QIcon icon  = berry::AbstractUICTKPlugin::ImageDescriptorFromPlugin(
                                         (*cmActionsIt)->GetContributor()->GetName(),
                                         QString::fromStdString(cmIcon));
contextMenuAction = new QAction(icon, QString::fromStdString(cmLabel), parent);