Always parse GTK/GDK/GSK_DEBUG env vars and make some entries available in non-debug mode

Currently GTK can be built with G_ENABLE_DEBUG which enables various debug code and parsing of those env vars, or without, which instead of parsing them prints a warning if they are set. While building with G_ENABLE_DEBUG isn't strictly needed it's the only way to make GTK_DEBUG=interactive work, which is a nice thing to have always.

This enables parsing of those env vars in any case and allows specific values being marked as also available when not built with G_ENABLE_DEBUG (interactive for example). If not built with G_ENABLE_DEBUG then all unavailable values will be marked as such in the help output and a note is added that GTK needs to be built with G_ENABLE_DEBUG to use them, which should help discoverability.