Draft: sharefileitemaction: Deparent menu in case parentWidget gets deleted before plugin

In system/dolphin!196 (merged) we want to choose the DolphinContextMenu as a parentWidget, but that gets immediately deleted. The plugin instances should still be kept alive for them to be able to report errors.

If we delete the parent the children would consequently be deleted too, but in this case we keep them around as member variables. That is why the menu gets depareted when the parent is deleted before the plugin is.