Avoid deprecation warnings caused by GTK_ALIGN_BASELINE_FILL

Michael Catanzaro requested to merge mcatanzaro/#5875 into main

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)

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