Introduce GTK_ALIGN_BASELINE_FILL/CENTER
It turns out that the old behavior of GTK_ALIGN_BASELINE was actually used in libadwaita, so bring it back, and introduce a new GtkAlign value for the new behavior.
It turns out that the old behavior of GTK_ALIGN_BASELINE was actually used in libadwaita, so bring it back, and introduce a new GtkAlign value for the new behavior.