Add a (diagnostic) warning for finalized floating objects

GTK currently checks if a GtkWidget is finalized while still using a
floating reference—i.e. a widget was disposed without any parent
container owning it.

This warning can be useful to identify and trace ownership transfer
issues in libraries using initially unowned floating object types.

To avoid introducing constraints ex post, we can gate this check behind
both the G_ENABLE_DEBUG compile time flag for GLib, and behind the
G_ENABLE_DIAGNOSTIC environment variable run time check.

Fixes: #2489
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