applets/systray: port to a nested containment

Make the Systray a nested containment

requires libplasma!1244

The systray containment can now live directly inside the panel containnment, while retaining its Containment capabilities.

Its only changes should be in the metadata file, to be renamed as the public facing applet and lose the "NoDisplay" key

The "systray container" applet that was used for the embedding can now be removed