Bug 612373 - Add "Span" option for Windows and Linux to "Set As Desktop Background". r=Gijs

Gnome introduced this in 2.30[0], Windows in 8[1].
Preview is just an "international prohibition symbol" to save the complicated
business of trying to calculate a semi-accurate one.

[0]
8eb0886312
https://bugzilla.gnome.org/show_bug.cgi?id=603551
[1]
https://superuser.com/questions/1061416/which-version-of-windows-introduced-the-span-wallpaper-style

Differential Revision: https://phabricator.services.mozilla.com/D23336

browser/components/shell/nsWindowsShellService.cpp

@@ -540,6 +540,10 @@ nsWindowsShellService::SetDesktopBackground(dom::Element* aElement,
         style.Assign('6');
         tile.Assign('0');
         break;
+      case BACKGROUND_SPAN:
+        style.AssignLiteral("22");
+        tile.Assign('0');
+        break;
     }
 
     rv = regKey->WriteStringValue(NS_LITERAL_STRING("TileWallpaper"), tile);