Seahorse + GtkBuilder

[Stef Walter <stef-list memberwebs com>]

Seahorse now uses GtkBuilder :)

I've committed the GtkBuilder from Andreas Brauchli. I've tried to go
over everything, to make sure no breakages were introduced, but since
this is a massive patch, I'm sure one or two got through. We'll need to
file bugs and/or fix the problems as they come up.

In the future do we want to handle massive patches like this as a branch
in the main git repository?

Cheers,

Stef