Allow scrolled windows to grow when the PGP key properties window is resized

Some containers weren't set to expand=true fill=true, so ultimately
the child scrolled windows weren't able to grow.

Also, make the combobox for trust *not* growable, so it doesn't
expand weirdly when the window is resized.

https://bugzilla.gnome.org/show_bug.cgi?id=727850