Create a reference to a GnomePnpIds object to speed up loading

During loading, the singleton GnomePnpIds is created and destroyed 3 times
during startup. This means that the pnp.ids is loaded from disk each time and
re-parsed.

Whilst loading it from disk subsequent times is fast, parsing is not.
By keeping a reference at daemon startup ensures it's only loaded and unloaded
once per daemon startup-shutdown.

This speeds up startup of gnome-settings-daemon by about 400ms.