Avoid deprecation warnings caused by GTK_ALIGN_BASELINE_FILL

We have to be careful to only use GDK_ALIGN_BASELINE_FILL when permitted by GDK_VERSION_MAX_ALLOWED because gtkenums.h is a public header. If it's not allowed, then just fall back to the previous definition of GtkAlign since they are both compatible.

I don't think we can avoid conditional compilation here, because the old definition is going to cause deprecated declaration warnings unless you define an old GDK_VERSION_MIN_REQUIRED.

Fixes #5875 (closed)