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.

Some of this code is by Florian, who helped with promisifying. Thanks!

https://bugzilla.gnome.org/show_bug.cgi?id=734996