Previously, they were still using blue.

Previously we've just hard coded the location to the fedora
logo.  Now it's a configure time flag.  We also have a configure
time flag for a background color that goes with the logo, but
none of the splash plugins make use of it yet.

We want the throbber to know about the window it's in,
so that it will be easier to move drawing code to a
draw handler model

This is a first step toward making the window provide
an draw handler interface, so we can hide the details
of pause/unpause, resetting raw mode, and graphics mode,
etc from the individual plugins.

Previously we were setting up the exit handler at the top
of the show vtable function.  This meant having to remove
the exit handler if the show fails. It's much easier to
not set up the handler until after we know we're going to
succeed.

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.