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.

We may want to do another convenience lib for plugin
specific apis, or fold window into libply

There was a sort useless layer of indirection between the
window object and splash plugins.  It ended up with
functions like:

void
on_backspace (ply_splash_plugin_t *plugin)
{
  plugin->interface->on_backspace (plugin);
}

Since the individual plugins are aware of the window object
anyway, they can register their own on_backspace et al handlers
without going through the ply_splash_plugin_t layer.