Require gtk+ >= 3.9

So that we can make use of baseline alignment API.

https://bugzilla.gnome.org/show_bug.cgi?id=698127
Require Vala >= 0.21.1

https://bugzilla.gnome.org/show_bug.cgi?id=698127
unattended-installer: Add baseline alignment to setup screen

Added baseline alignment to username (label and entry) and
password (label and notebook).

https://bugzilla.gnome.org/show_bug.cgi?id=698127