Delay login animation until after wallpaper has loaded
Currently, the login animation can occur before the user's wallpaper has been loaded. When this happens, we wind up displaying a solid blue background for half a second or so before the proper background is displayed. This looks jarring and bad. It's great that we can start GNOME quickly, but starting up before the wallpaper is ready is too quickly.
I've been meaning to fix this since 2014. Better late than never! We can just have BackgroundManager emit a loaded signal the first time it loads its first background, and have the startup animation code wait for that before proceeding.