gnome-ask-password-agent.vala: check for VALA_0_12

gnome-ask-password-agent.vala: check for VALA_0_12

<juergbi> VALA_0_X is defined up to and including the current version
<juergbi> so VALA_0_10 is defined in 0.10 and 0.11 and 0.12
<juergbi> VALA_0_12 is defined in 0.11 and 0.12 (and later versions)
<juergbi> reverse the branches and use VALA_0_12 as conditional
<kay> juergbi: changing api but keep the defines?
<kay> how do we support vala 0.14 then?
<kay> you keep that model with the old defines?
<juergbi> VALA_0_10 corresponds to valaversion >= 0.10