This will be important for determining the propery location
of the throbber on screen

The old approach of including the same source files into
multiple places is less than pretty.  But moving the code
to libply didn't seem right either, since libply is fairly
generic and could in theory get swapped out with a standard
runtime at some point.

Also, exposing libplybootsplash as an external library will
facilitate creating out of tree splash plugins.