Implement align_label for GTK 3.[14,16).

Implement align_label for GTK 3.[14,16).

gtk_misc_set_alignment was deprecated in GTK 3.14. But my replacement
code using gtk_label_set_{x,y}align doesn't work there, because that
function wasn't introduced until GTK 3.16, so there are two minor
versions in the middle where a third strategy is needed.

(That middle strategy doesn't permit arbitrary float alignments, but
that's OK, bceause we only actually use multiples of 0.5.)