widget: Set the background alpha in set_colors

We are doing that in vte_terminal_set_color_background, but a lot of
applications (eg., the test application, gnome-terminal) use
vte_terminal_set_colors. So set it there as well.

Fallout from 060850078a53dce64c3d2b96882f525fef7f5028

https://bugzilla.gnome.org/show_bug.cgi?id=729884