Introduce ContainmentDisplayHints

The containment may want to give plasmoids some extra information on how to look and behave, which the plasmoids can chose whether to follow or not. this adds the two hints ContainmentDrawsPlasmoidHeading for containments that draw headers to the plasmoids as a kind of "server side decoration" which will tell plasmoids to not try to draw their own (and if they do their heading, it should be hidden in that case)

Another is DesktopFullyCovered that informs applets that the desktop area of the containment's screen is not visible and fully covered by something, for instance a maximized window