There's an obvious inconsistency in this patch between what I did
to AppDisplay where I added an actor property that mirrors 
AppDisplay._grid and to SideShow where I renamed AppDisplay._group
to AppDisplay._actor. This basically reflects indecision, but
the renaming seemed less obvious for AppDisplay where the information
that the actor was a grid was more relevant than the information
that the actor was a generic group.
===
appdisplay.js: Add AppDisplay.actor property, remove the x/y parameters
  to the constructor, and don't automatically add the appdisplay
  to the stage.

overlay.js: Make the AppDisplay a child of the SideShow, rename
  SideShow._grid to SideShow.actor and slide the SideShow in when
  showing the stage.