greeter: Assign "wallpaper" context property earlier for plugins

Some wallpaper plugins, such as the Shader Wallpaper on Pling, make use of the wallpaper context property in bindings and on component completion. Assigning it only after the SharedQmlEngine finished loading is too late and can result in numerous QML reference errors.

Move assignment of the new SharedQmlEngine's context property further up to be called before component completion, with only the containing view getting hooked up after the delay.