Introduce G_ENABLE_CONSISTENCY_CHECKS

Introduce G_ENABLE_CONSISTENCY_CHECKS

Use a separate G_ENABLE_CONSISTENCY_CHECKS define to guard internal
consistency checks that are applied unconditionally if they are enabled,
such as the widget invariants checking. Interactive debug spew that can
be triggered at runtime with the GTK_DEBUG environment variable is still
guarded by the G_ENABLE_DEBUG define.

The mapping from enable-debug levels to defines is as follows:

yes:     G_ENABLE_DEBUG G_ENABLE_CONSISTENCY_CHECKS
minimum: G_ENABLE_DEBUG G_DISABLE_CAST_CHECKS
no:      G_DISABLE_CAST_CHECKS G_DISABLE_ASSERT G_DISABLE_CHECKS