PlasmoidHeader: Make it based on T.ToolBar, deprecate location
PlasmoidHeader was probably based on T.Frame because of a mistake (@niccolove, can you confirm?). T.Frame doesn't do anything different from T.Pane and PlasmoidHeader was duplicating the function of T.ToolBar's position property with the location property.
I talked to @mart in the Kirigami chat room already and he thought this was a good idea.