Settings panels should not be displayed in the overview

[Michael Catanzaro]

In Ubuntu GNOME 14.04, gnome-control-center settings panels appear as programs in the GNOME shell overview. There are almost as many settings panels here as there are real programs, making it significantly more difficult to use the overview to find real programs. Other GNOME distros do not include settings panels in the overview as of [1], which fixed this by adding NoDisplay to all the panels' desktop files. (It's true that other distros also have a search provider to find the panels, which Ubuntu's old gnome-control-center does not provide, but I do not think that justifies polluting the overview.)

[1] https://bugzilla.gnome.org/show_bug.cgi?id=712246